interference question
 

quick question for me about my stereo. So i wired a new head unit and an amp and a sub, all that stuff works great (thanks to rover chris's wiring diagram and cutting into the speakers at the amp harness). I tried replacing the 6.5"s on the back gate with a 3-way 6.5" and everything works great, i ran it off channel three of my amp and when the truck isnt RUNNING it's got great sound, but when the truck is running, theres a pretty loud whine coming from just that one speaker.
What could cause switched interference like that?
Just to try and answer any setup questions that may come first, im running 20ft rca's from the back of the head unit (preamp rear) into the amp, theres no L/R on the amp, only one input for each channel, so the rca's are plugged into channel 3 and 4's inputs. From the back of the amp, im running reg speaker wire over to the 3-way on the door. Im not bridging or anything. do you think i have a bad connection at the head unit? Idk what would cause the interference only when the truck is on.

Best

Turf

If whine changes pitch with rpm, probably the alternator noise getting in there somewhere. Might want to double check your grounds.

im gonna go back after it now and give the head unit a new ground and replace the most recent RCA cables with more expenisve shielded ones. The pitch doesn't change with RPM, thats a good point. So if it's not the cable shielding or the ground (which im pretty sure already it's not) then i'll be totally stumped. Electric is so confusing!

New rca's didnt help, went back to the original, here's my new ground, brought straight from the back of the head unit harness, joined to solid wire and fixed to the firewall
Attachment 43958

Here's what i learned today. The noise IS affected by engine RPM (you were right), i.e. throttle up-->whine up.
the noise DOESNT matter what channels are plugged in, when i run the 6.5 off of channel 3 it whines, when i run it off channel 4 it whines.
When i only plug in one RCA to the back of the amp for channels 3&4 (for the channel running the speaker) i get no sound, when i plug in the one that doesn't have the speaker run off of it, i get noise.
Is there anything else diagnostic wise i can do to this thing? I'm pretty well stumped. I'll re-check the amp ground tomorrow, but it's done similarly to the head unit ground.
Is this something i could take to an indy shop and pay them to fix? Or would i waste all my money on them trying to find the problem?

If i find out that disconnecting the alternator stops it, whats my fix? What do i need to shield?