Electronic Noise

It could be radio noise from DC to AC converters (inverters) that some truckers use.

Good answer. Most likely

Next time I will pay more attention to the material on the side of the trailer.

What do you think the reflections are from?

What would be the source of what's being reflected??

Probably the remnants of a supernova, many light-years away. That is, if you were driving East at the time. If you were driving North, then it was Mildred's old Electrolux vacuum cleaner.

Not Just Mildred's vac.

I have a new Philips Air Purifier and that intereferes with AM and FM as an irritating buzz

That looks eerily similar to my portable jet-pac.

How about static? Maybe the truck is poorly grounded, and you are getting the noise from static discharges? Remember when gasoline trucks were grounded with chains dragging? Therefore, static potential does build up and must be drained somehow.

That is what I was thinking. Some tires are bias ply, some are steel belted which affect the conductivity of the tire. Static could indeed build up.

The mud flaps could also create static.

And as was mentioned, the source could be your vehicle ignition system with noise from spark plugs or wires and the noise being reflected from the truck.

Open your hood at night, with engine running and look for sparks along the plug wires or a purple glow.

Dirty brushes in the alternator could also be another source, as could any loose/dirty connection of the electrical system or a loose connection from the truck frame to the trailer.

I remember the days of polyester tires, and on a dry day, when you exited the car, the static would snap you out of any daydream you were having. People started putting a leather strap from the frame to the ground and that solved the problem.

Of course, if your wife is sitting beside you ,you are liable to be getting static from her.

Next time it occurs, notice if the highway is concrete or asphalt.

Is it worse in dry weather?

What is the distance from the truck when the static is heard? Does it change with the distance.

Feed back from you will be greatly appreciated.

Air traveling over paint can create static too.

OK - little more 'research' this week.....

I live on the border of two states in the US. When I had posted this I had traveled west and experienced the noise both going and coming home

Yes, I am somewhat ancient listening to AM radio, but it is for national and local news. Otherwise I have a flashdrive plugged in that will play some really rocking blues!!!

But, this week I drove east into the other state - no noise passing any truck! Same on the way home. Two trips this week.

Are we looking at pavement ingredients and friction of the tires??

Otherwise, same atmospheric conditions - speed - time of day....... Just curious.

Interesting, but comparing a one-off is not conclusive, you would need to do at least a couple of trips East West to see if the pattern actually exists.
If it does only occur when you travel west, then I would be looking for some kind of fixed structure and not a moving truck, as you passed trucks in both directions I'm sure.
Does the interference only occur with a particular radio station or does it still occur even if you change stations ?
Maybe if you travel West and you get interference try and stop and see if the interference continues after trucks have passed or change radio stations.
Power lines can also cause AM interference, maybe see where they are located in relationship to the road or maybe a Power station.

An increase in the amount of carbon black in all tires has mitigated the static charge concerns.

All trucks or some trucks? Check the diodes on the truck alternator. Then rf bypass truck charging cable from alternator to battery.

Do the high noise trucks have CBs?

Are there power lines along road? Dirty insulators on towers?

Find a quiet spot on roadside, park and listen to passing trucks.

Hmnn AM Radio ?? man you live in the Dark Ages, I can't even remember when I last used AM and LW passed away long ago.
Today I use DAB where possible or FM.
So this interference only occurs sometimes when you pass a Semi, the chances are highly probable that as Haymaker pointed out, the electronic noise is generated by a CB Radio.
Way back in the 70-80 ties when I was driving semis, in Europe CB radios caused interference, especially when a "burner" was being used.
Depending on location (high up in the hills / mountains) and "burner" power one could talk to truckers in the US from Europe and a friend of mine used to talk to a woman in Brasil on his CB Homebase.
One cause of CB interference is badly tuned Antennes. Either Amplitude Modulated (AM) or SSB (Single Side Band) caused a lot of interference.

In Europe CB's are restricted to certain transmitting power / range
As the distances in the USA are far greater than densely populated European roads the chances are more likely that the US truckers have a "burner" wired in !
Simple test, drive round a semi parking lot at a truckstop, put AM radio on and see if you get interference. If you then get interference chances are you have your answer.

I've noticed this a lot since the advent of electronic drivers logs. I suspect that the trucks are also being tracked by GPS systems to monitor speed, location, etc.

I'm sure these vehicles are transmitting data to a satellite or something. It IS annoying.

I've also noticed this a lot. I work near a port with many dozens or even hundreds of trucks moving or waiting on the roadway/terminals.

I almost only hear this when traveling down the road. Most times, the parked/waiting trucks do seem to emit this heavy interference but they are idling or turned off or in some reduced load mode.

The static is severe but I do remember some stations on the AM band being less affected. Now I have to experiment on whether lower or upper bands are more effected. FM is not affected from my remembrances.

You all have some great ideas on the cause but unless we have access to a truck to test this is fun speculation.

I always assumed it was poor connection to the trailer or the CB radio as some are hacked to produce more power as that is a popular thing to do in the past before cell phones.

Its exactly when they are switched off or Idling and the driver has nothing to do except chat to other truckers or sometimes bored housewives ! Thats when the usage goes up and the normal CB is limited to 4 watts output, with a tuned rig and a good antenna, range in flat areas upto 12-16 miles, however if the driver is running an illegal burner up to 1000 watts plus he can talk clear across the states if hes up in the mountains and has a clear transmission area, but he will be generating a lot of inteference.
Go talk to a trucker or two, they don't bite, (usually ) they'll be glad to tell you about their Rig including the CB radio.

Amplitude Modulation is far more affected by disturbances than its younger replacements.

The receiver cannot ignore the noise because the signal is conveyed by the changes in the peaks which is influenced by outside sources.

There are many sources of noise the receiver will pick up. Electronic noise from the generator , spark plugs or outside sources even solar flares.

AM signals can be reflected . the transmitter stations could then be spaced wider apart where FM require almost line of site.

A suppressor (capacitor) on your power supply may filter out some of the noise.