Ok, so this problem has been getting on my nerves for the past few months lately. It seems that, depending on my speed, the volume of my stereo when it is at a solid level, tends to decrease in volume or increase. Usually when I slow down it gets quiet, and speed up, it gets louder. What could be the possible problems or how can this be fixed? Is there some kind of speed sensor that monitors the engine noise that creates this even with an aftermarket radio? Thanks.

bump. anyone?

yes, speed sensor.

I forget the fix but it is in your radio manual.

also if you search there is a thread on which wire is the speed sensitivity wire.

best of luck

It may also be capacitor, do you have it installed?

no, on that car there is a speed sensor. US cars are a little different.
BMW Fix - unplug that little wire in the back of the radio that "nobody knows what it does"
My fix - throw away garbage c33 radio. get something nice.