So i finally got around to putting a 2m/70cm mobile rig in my truck, been meaning to do this for like two years now. Went with a Yaesu FT-7900R and a NMO mount drilled through the roof. Actually pretty happy with how it turned out except im getting some weird feedback or maybe its just desense? Hard to describe - when someone keys up on the local repeater theres like a slight warble right at the beginning of their audio sometimes. Happens maybe 1 in 5 transmissions.

The antenna is a dual band mag mount, wait no i drilled a hole so its a proper NMO, i have a Diamond NR770H on there. Coax run is maybe 8 feet to the radio under the seat. Grounded the radio chassis to a bolt under the dash.

Also wondering if where i put the antenna matters much - its roughly center of roof, maybe slightly toward the rear. Truck is a full size F-150. Figured center was best but now reading stuff online saying slightly off center forward is better? Curious what people think. The repeater coverage seems fine so maybe its all fine and im overthinking.

Congrats on the install, NMO through the roof is the right way to do it. The warble thing you're describing sounds like it could be a few different things - desense is possible if the antenna is physically close to any other antennas or if the coax has a nick in it somewhere, but honestly on a 2m/70cm setup with that radio and antenna combo desense would usually be more obvious than just a warble on 1 out of 5 transmissions.

My first thought is RF getting into the audio somehow, which can happen if the chassis ground isn't great. That bolt under the dash - is it actually grounding to the vehicle chassis or just to the dashboard structure? On newer trucks that stuff isn't always directly bonded to the main chassis. Worth running a short fat wire directly to a known good chassis point. I had a similar warble issue in my old Tacoma and it turned out to be a bad ground causing all kinds of subtle weirdness.

As for antenna placement, center of roof is pretty much ideal for mobile VHF/UHF, you'll get a nice even pattern all the way around. Slightly rear of center is also fine. Front placement would favor the forward direction a bit which some people like for highway driving but honestly for working repeaters it really doesn't matter much. Sounds like your install is solid, don't overthink it.

yeah the grounding thing is real, i chased a weird audio issue for weeks and it was just the chassis ground. also fwiw ive got an FT-7900 in my jeep and love it, great radio for the price. what repeater are you hitting if you dont mind me asking? sometimes that warble is just the repeater itself or someone with a marginal signal hitting it from far away