After having removed the incorrectly installed aftermarket radio (wiring was completely incorrect) out of my E30, I put back in the stock radio that came with my car when I bought it and wired it back to stock. After turning the radio on I noticed a terrible crackling and popping sound when I would touch or try to adjust the volume. The fix was surprisingly simple. As far as I am aware this applies to all E30 radios, and many other radios of the same generation (e28, e24, etc)
The stock volume knob is a Potentiometer; when you turn the knob, a wiper goes over a restive material which then controls the volume. Over 20+ years of loyal service, stuff starts to get gummed or ditry in there, thus creating that crackling/popping sound.
How to fix it:
There is no need to remove the radio. Go to your local auto store and buy electrical cleaner, pull the volume knob off the radio. There is nothing retaining the knob it just pulls off. Spray a liberal amount of electrical contact cleaner in the hole, on the shaft, and behind the shaft. Then put the volume knob back on and turn the volume up and down numerous times, also remember to have the power off. Thats it, your radio now works like new!
Likely this crackling sound is what made the PO of my E30 change the head unit. Thankfully it was included with the sale and I was able to put it back in.