We have a motorhome and were getting engine noise through speaker

Go to Radio Shack and purchase a radio filter which is called a noise compressor

You have grounding issues with your stereo, you might want to run a separate wire from the battery ground to the stereo, or you just might need to clean the battery cables. do yourself a favor and dont pour coke on you battery poles, its a quick fix but before too long the sugar just carmelizes and causes even more issues, just take em off and clean them, and if that doesnt help look to the stereo grounding wire, you could even run one to the metal on the chassis and will improve it

Ok,,,this is an easy one to fix,,,
its a cracking or screaming sound ,,,right?
its off the car spark plugs and from the plug leads,,,
you havent fitted any serpresors to the plug leads
you can do one of two things here buy new plug leads plus the coial lead but they must be suprested leads,,,or just buy the suprested caps and fit them yourself,,,its best to buy new leads,,,when you fit them only pull one lead off at a time and fit the new lead to the one you pulled off that way you cant get in a mix up,, its very unlikely the alternator will cause a buzzing but they can at times,,,check that the radio ariel is earthed to the car body,,,it must be earthed!