so i've been fighting this install for like three weekends now and i think i finally got it dialed in but my buddy says my audio sounds kind of muffled and occasionally has this low hum underneath it, not terrible but noticeable enough that he mentioned it twice on the way home from the repeater meetup last night

i've got a yaesu ftm-400 mounted under the dash on the passenger side with a diamond NR770H on the roof, ran the coax down the a-pillar and under the carpet. power leads go direct to the battery with an inline fuse about 6 inches from the positive terminal. ground goes to a bolt on the chassis under the dash which i sanded down to bare metal before connecting

the hum is worse when the engine is running which makes me think alternator noise maybe? and the muffled audio im not sure if thats a mic thing or settings. the mic is just the stock one that came with the radio, nothing fancy. i checked deviation on the radio and it seems fine but honestly im not 100% sure what im looking for there

anyone dealt with this kind of thing? i dont want to rip everything apart again if i can avoid it

the hum that gets worse with engine rpm is almost certainly alternator whine, classic symptom. few things to check before you tear anything apart though. first, how long is your ground lead? if its more than like 12-18 inches you might want to run a dedicated ground all the way back to the battery negative instead of chassis grounding it. sounds counterintuitive but chassis grounds can be really inconsistent depending on the vehicle, especially newer ones with all the electronics in them

also check where your coax runs relative to the power leads. if they're bundled together for any length of run that can cause all kinds of noise to get into the audio chain. try separating them and see if the hum changes at all, its a quick test that doesnt require pulling anything apart

for the muffled audio, first thing id do is just check the mic gain setting in the menu, a lot of people dont realize the ftm-400 ships with it set pretty conservatively. also make sure the mic isnt partially blocked by the mounting position, ive seen people mount the head unit where the mic ends up pointing into the dash and it just sounds terrible

yeah alternator whine for sure, had the exact same thing in my truck with a 2m/70cm setup. ended up using one of those icom noise filters on the power lead and it cleaned up almost completely. they're like 15 bucks on amazon and they're super easy to inline, totally worth trying before you go nuts chasing grounds

the NR770H is a solid antenna by the way, good choice for a dual bander. what repeaters you running on mostly? we've got a pretty active 70cm machine here that i'm on most evenings if you want to do some signal tests once you get the noise sorted