>>I have been told that when I am driving at speeds of 15MPH and under, I am giving off some engine noise.<<
If you mean that others are giving you signal reports that there is engine noise on your transmitted audio -- and you are talking about a diesel truck, then it could be alternator whine, injector noise, or possibly fuel pump noise.
One thing to try that would be easy is to place a power inductor as filter right behing the radio on the positive wire in situations like these.
You are in luck these days, due to the proliferation of idiots running super load boombox amplifiers in their cars, we no longer have to search high and low for such a filter that can handle better than 20 Amps, nor do they cost an arm and a leg anymore, due to imports because of that customer demand.
DC Powerline Noise Suppressor, 35A
mcmelectronics.com sells one for $18.83 MCM Part #: 60-1270 -- likely you can find one anywhere those boomboxes for autos are sold out on the road, too. http://www.mcminone.com/product.asp?catalog%5Fname=MCMProducts&category%5Fname=3829632&product%5Fid=60%2D1270
Attached to the radio, it could even move to another rig or car with you.