First, determine if the sound is related to audio - or signal.
Have someone listen to your AM radio transmission nearby. With an un-modulated carrier, the signal should sound clear. If there is noise on the signal, try with a different mic. If no mic is available, key up the radio by shorting the PTT pins on the jack on the radio itself (this will remove the mic element from the circuit - effectively disabling any audio noise entering from the mic cable or element. If the noise disappears or is significantly reduced, the problem lies with the mic, cable or connections at the mic plug or the inside the hand help portion.
If there is no problem in the above test - then the trouble can be a result of the signal. Signal issues are more pronounced when there is more space between your transmitter and the other person's receiver. If they are close to each other, it will be difficult to hear problems. Things that can affect signal quality - and noise can be: 1) vehicle electrical systems. See if there is a noticeable difference with your signal compared when transmitting with the engine on and engine off. Fuel pumps can be a source of noise, as can ignition noise, alternator whine, etc. A "choke input" filter or a large value microfarad capacitor rated at more than 15 VDC installed between + and - wires can help mitigate these noise sources - but it is unlikely that either will eliminate it. 2) power wiring. Always get power directly from the battery. This is the cleanest source of power available in the truck. You can interrupt the 12+ line with a relay that is controlled by the accessory terminal if you want the radio to shut off when the ignition key is removed it you wish - but resist the urge to use the accessory terminal to power the radio directly. 3) Antenna issues. Dual antenna installations require 75 ohm coaxial cable (co-phase harness) to perform properly.
These can be as simple as pinched coaxial cables to improperly tuned antennas. Damaged cables need to be replaced. High quality, low loss, 75 ohm coaxial cable is required for the demands placed on it in mobile situations. Tuning for the lowest SWR or VSWR is a necessity from effective power transfer from the radio and antenna coax to the radiator or antenna. A detailed description of how to can be found here
. Be sure to keep in mind the last sentence in the article regarding co-phased antennas: Adjust BOTH antennas to keep them in balance.
Hopefully, one or more of these suggestions will help solve or reduce the problem you've got. Good luck!