radio noise in truck stereo

I have a pm400 vhf that was in my 01 explorer and it was used with no probs for about 4 yrs. I put the radio in my 01 Silverado and when I key the mic, I get a loud popping humming sound through my sound system that I never had in the explorer. the antenna is mounted on the roof via nmo hole mount, radio is mounted on my headliner console, grounded about 12" from it, power cable is spliced into my 4ga amp wire (maybe the prob?) that comes straight from the battery. what do yall think if I run my power cable to the radio directly from the battery or another good 12v power source? thanks for any help in advance!!