Here the break down of whats happening, I recently had a custom install put into my Riviera. This is by no means my first large system, but has by every stretch been the most problematic I have encountered.
So far my major issue that we haven't solved is a violent crackling followed but cutting out on my trebile speakers after I exceed just over 50% volume. My car is a uni-body but all power supply wires and RCA's etc. are on apposing sides so that rules out interference from the power supply to the amp. ! issue we fixed was 1 of my tweeters and crossovers had to be shielded with aluminum foil to stop it from picking up ECU noise even when it wasn't hooked to a power supply. That has been corrected but still have the crackling. Most bizarre I must say. Its also affected by me stepping on the accelerator or brake pedal? . Alternater tested fine and I can only assume my ECU grounds are good or I would be experiencing far more problems than speaker noise. Please help me find this gremlin guys, thanx in advance
like above, check your grounds. especially the head unit. Also check the head unit power. If it is an older car the power might not be enough so when you turn the volume you clip the signal.
Worst case scenario, install Noise suppressors (ground loop isolators) between the head and amplifier(s). I have a buddy that is going through the same thing right now...we've checked, double checked everything and suppressors were the only thing that worked. We even ran new ground and power lead for everything bypassing the vehicle's OEM electrical system completely.
Some cars just seem to not like after market systems...