Testing a Uniden 980 SSB with a RM Italy KL 203

I'd say the modulation is set to high, but it never made any buzzing sound in my Jeep with the 6 foot skip shooter antenna. I'm guessing just some strange harmonics going on while hooked up to bench power and dummy load. I've tested many amps that would run perfect in the mobile, but showed strange operation while on SSB on the bench, while others ran perfect. Not really sure.