so anyway ive been messing with this install for like three weeks now and i thought i had it all sorted out. got the FT-857D tucked up under the passenger seat with a ram mount for the head unit on the dash, ran the power leads directly to the battery with an inline fuse, everything looked clean. but now whenever i key up on 2m or 70cm i get this horrible buzzing in the audio that kind of pulses with engine rpm, which makes me think its alternator noise getting in somewhere but i already put a ferrite on the mic cable and that didnt really help.

the coax run goes from the antenna (NMO on the roof) down the A pillar and under the carpet to the back of the radio. i used LMR-240 for most of it. ground is on a bolt on the firewall. not sure if thats the issue or if its something with the coax routing being too close to the power wires at some point. anybody dealt with this before?

alternator whine like that in the audio is almost always a grounding problem in my experience, even when you think the grounds are good. the firewall bolt sounds fine but id double check that you have a really solid connection there, like actually scrape the paint off down to bare metal before you put that ring terminal on. also the radio chassis itself needs its own dedicated ground, not just relying on the power cable negative. some guys also run into trouble when the coax shield isnt grounded at the antenna end properly, worth checking if your NMO mount is actually making good contact with the roof metal. the LMR-240 is fine, that shouldnt be causing issues.

one other thing, try the radio on a totally separate battery outside the car just to confirm its actually RF/alternator noise and not something internal to the radio. narrows it down quick.

i had almost the exact same thing with my 706 years ago and it drove me crazy for months. turned out my power wires were running parallel to the coax for about 18 inches under the carpet and that was enough to cause problems. separating them fixed most of it. also ended up putting a cheap noise filter on the power line right at the radio, one of those astron or similar inline ones, and that got rid of the last of it. not the most elegant solution but it worked fine for years after that.