Make sure that all audio jacks are insulated from the airframe with dielectric
washers. Check electrical system for ground loops.
A dead rectifier diode in the alternator will do that. Load up the alternator as much as you can. One with a bad diode can't make full rated current. The max is more like 45A instead of 60+.
Another thing to check is the airframe to engine ground strap along with the headset jack isolator washers suggested above.
Not difficult to remove your alternator & take it to a local Auto Electric shop & have them test the Diodes, they probably have the part there, standard ND part.
Would save major $$ compared to replacing with another PP unit, or as is likely to be suggested, a new B&C unit.