Something wrong with my amp!!!?

I've been playing electric guitar for a year now and when i set my amp to overdrive with the gain on maximum the amp starts to have some sort of feedback noise? Could you tell me why? My amp is a Kustom ( small and crappy ), and my guitar is strat copy ( the pickups are a bit rusty ) and i'm using a golden tip cable. If you know why this is happening please tell me. Thanks for your time.