so ive been putting off doing a proper mobile install for like two years now and i finally just did it last weekend. got the FT-857D mounted under the passenger seat with a Diamond NR770HB on the roof, ran the power cables directly to the battery with an inline fuse, pretty much followed the standard advice ive seen on here a hundred times.

thing is now when im driving theres this sort of whine or buzz that changes pitch with the engine RPM. its not terrible but its noticeable enough to be annoying on 2m and its worse on UHF for some reason. radio works fine parked with the engine off. i did separate the power and coax runs on opposite sides of the vehicle so im not sure what else i might be missing. anyone dealt with this before? is it the alternator? i feel like it probably is but wanted to ask before i start pulling stuff apart again

yeah that's classic alternator whine, the pitch changing with RPM is the dead giveaway. happens to almost everybody at some point. a few things worth checking -- first make sure your power lead ground is going to the battery negative directly and not just to the chassis somewhere random. even a short chassis ground run can pick up noise. also check if your coax shield is grounded at the antenna mount point, if the mount isnt making good contact with the roof metal it can act like an antenna for that interference instead of rejecting it.

if the grounds all check out you might want to try a ferrite choke on the power leads right where they enter the radio. just loop the cable through a snap-on ferrite a few times and see if that cleans it up. sometimes you also need one on the mic cable too which seems weird but it works. ive cleaned up some pretty bad installs that way without having to rewire anything.

had the exact same thing in my Tacoma with a 7100, drove me nuts for weeks. ended up being that my antenna mount wasn't grounded properly to the body -- there was a clear coat patch right under where the mount sat so it wasnt making metal-to-metal contact. once i scraped that down and remounted it the whine dropped like 80%. still had a little left and the ferrite bead trick on the power line got rid of the rest. good luck, its annoying to track down but its usually something simple once you find it