so ive been running a 2m/70cm mobile setup in my F-150 for about 6 months now and i keep getting audio feedback or what i think is RF getting into the cab audio somehow. its not terrible but its noticeable especially when i key up on 70cm. the radio is a Yaesu FTM-400 mounted under the dash, coax runs along the firewall and up through the door seal area.

the antenna right now is a mag mount NMO on the roof toward the back. i know mag mounts arent ideal but i havent wanted to drill yet. wondering if anyone has dealt with this on a pickup specifically and whether moving the antenna further back toward the cab-over-bed area would help or if this is more of a grounding issue. the truck has the composite bed liner and i wasnt sure if that matters for ground plane stuff.

also my SWR on 2m is fine, reads under 1.5:1, so i dont think its the antenna itself. just something feels off about the installation and i want to clean it up before winter.

had almost the exact same thing on my old Tacoma. turned out the coax shield wasnt making great contact at the mag mount base, which was causing RF to travel back down the outside of the coax instead of staying on the shield. classic common mode current problem. what fixed it for me was adding a few ferrite chokes on the coax run close to the antenna feedpoint. mix 31 toroids work great for VHF/UHF, you can slip the coax through a couple of them and it makes a noticeable difference. also worth checking how the coax is routed — if its running parallel to the power leads for any length that can cause issues too.

the composite liner shouldnt matter much for the antenna ground plane since the mag mount is on the metal roof of the cab anyway. if you ever do decide to drill, an NMO mount through the roof with a proper ground connection will be a big improvement over the mag mount regardless.

honestly might just be a bonding issue with the truck body. i ran into something similar and spent weeks chasing RF problems before someone told me to just bond the hood and trunk lid (or in your case the bed) with braided ground straps. made a huge difference on my install. not saying thats definitely your problem but its worth checking before you start moving the antenna around.