A car named 'Gravel' isn't part of the base game so you are probably talking about a mod (maybe even a Russian mod which are known for their poor quality). The base game models should have the best sounds, etc.
Tiny rocks hitting the undercarriage of your car. If your trashbag has tiny pieces hitting on metal or similar, then yeah
I personally really enjoy the new sound updates, and especially when i comes to dirt and so forth, the new sounds add more immersion which i very much enjoy. But apparently it's not everyone's fancy...
I will agree that the new sounds dont quite have the "crunchiness" of gravel at very low speed, but at higher speeds they sound great.
Yep, that's because the technical design only allows for a single looping sound event for all loose surfaces, and no multiple loops within that even, so traditionally you would have 2 - 3 loops which cover the speed range of that surface sound and the 'slow' loop would be 'crunchier'. We're then using the kickup system to give the audio feedback as to what surface you're driving on, and of course when driving slowly, the kickup is less.