Well, if it's following you around and present on many different radios including professional rigs, then I am inclined to think it might be caused by some other device you are using that you keep on you all the time (that is if we go on the assumption that it is not a widespread problem common to the whole area you cover). If it follows you while walking away from your car and using a portable radio then maybe it's some portable smart phone, tablet, or digital camera?
Try using one portable receiver and walking around with absolutely no other electronic device on your person in various areas and see if the noise decreases or ceases completely. If so, then go back and add one electronic device that you normally carry at a time to test when the problem recurs. You might find which device you use frequently is causing the problem.
I think you have stated in other threads that you are a news photographer or stringer or some such. If so, it could even be your professional photography or videography gear. Even things like portable lights, motorized zoom lenses, etc., might be spewing out lots of garbage that kills local RF, especially low band. Even when they are supposed to be "turned off" some internal smarts, say in your super fancy high level professional quality DSLR or even in your quicky backup point and shoot might remain on at some level and spew out enough RF hash to cause issues on your equipment.
Something else you could try is to sniff your equipment with a portable scanner held up close to each piece of gear to see which one causes the most hash. If possible, use AM mode to do this and turn the squelch completely down/off. Start with the antenna on and then remove it so that you have to be very close to the offending device to get the garbage so that you can zero in on the exact piece of gear. Of course, it could be coming from multiple devices so be ready for that too.
The reason you see the full scale S meter reading on your scanners but only hear the audio of the noise intermittently is that the squelch circuit is based on FM background noise (the hash you hear when the squelch is turned completely down under normal no interference situations). Many forms of electrical interference are broadband in nature relative to the receiver's specialized internal filtering. What gets through to the squelch circuit just looks like more of the same regular FM background noise so it gets muted. The S meter, however, is looking at different circuitry and actually looking at signal levels present within the final IF filter passband. So in FM mode the meter might show the presence of a signal but the squelch is still muting the audio. Since the noise from the interference is varying in quality, possibly including varying its bandwidth and has random phase, frequency, and amplitude changes there will be times that the interference alters enough to make noise like an actual FM (albeit distorted and low quality FM) signal so that the audio circuit unmutes and you hear the noise. Many types of digital gunk from computers, networking equipment, smart phones, cameras, etc., will radiate quite well and through quite a range of radio frequencies; they will often change in some way as the internal systems go through their regular digital routines.
That is why I suggested using the AM mode to sniff for the culprit(s). AM is much more prone to external noise so you will usually hear it better while in that mode.
-Mike