Engine noise problem. I think

it only happens when the car is running? does it get higher pitched or change sound when you rev up the car?  try this run rca.s from a freinds head unit  to your equipment. and see if the problem is still there? this will tell you if it is in your head unit or some whereafter.

it does sound like the hu. and if your handy with solder. take the radio apart on the bottom side of the main board axcessable from removing the bottom plate of the radio search the board for a burnt trace. when you find this repair it with a single strand of copper wire from a multi strand wire and solder it on that burnt trace after cleaning it with achol. so to connect that trace in the curcuit. if no burnt traces are found check the diodes in the power supply for the hu unit on the oppisite side of board if this is all forign to you go get another hu. lol