Okay, my CB knowledge is a bit 'dated' here....like from 1969 or so.
There are some easy ways to check it out.
What is the SWR reading? The closer to 1:1 the better and over 2.5:1 I'd get worried. Most antennas have a way to adjust the SWR or load. On the majority of the antennas I used (ground plane type), you adjusted the length of the mast to tune the SWR. Sometimes, you have to shorten the length but be very careful cut to much off and you'll need a new antenna.
The lower the SWR, the more efficient the transmit and receive. Also, ground planes on bikes, RV's and Semi's can be a trip.
http://www.firestik.com/Tech_Docs/CBs_&_RVs.htm
You probably were well aware of all of the above but, I figured I'd go over it anyhow.
The Mic, cable and connection can cause a lot of noise. If there is a small break in the shielding or a wire or a connection point, the impedance can change causing noise. Easily tested by swapping out the mic for a known good one.
The coax between the CB and the atenna cannot be kinked, smashed or have damaged connectors. This usually shows up in the SWR though.
I found this out on my rig. When I put a new high gain short mast on my trunk lid, the noise was terrible and the SWR was almost 3:1. I went around in circles until I asked my Uncle about it. He looked at it, opened the trunk and pointed to a pinched piece of coax right where the cable exited the trunk.
If the SWR is around 2:1 or so, I would substitute a mic first.
