I'm not an expert but it sounds like a problem with the grounding of the amp.
![](./images/smilies/spacer.gif)
you can buy something called a ground isolator i think.
![](./images/smilies/spacer.gif)
1st things first, try grounding the amp somewhere else, and check that there are no wires touching the bodywork anywhere
![](./images/smilies/spacer.gif)
(including the speaker)
![](./images/smilies/spacer.gif)
Thats about as far as I can go!!
![](./images/smilies/spacer.gif)
Hope it helps
![](./images/smilies/spacer.gif)
Steve