I got this Crate bx50dlx amp in a garage sale very cheap ($30) without testing it. It does work but after turn it on without any thing plug, I neutral nose hiss comes gradually after two or three seconds. You can still play but it is a little annoying. I thought it was related with grounding but I checked and changed the main cord and still is doing it. What could it be? Is it worthy to take it to the shop? it sells used for a little over $100 so I wonder if it makes sense to take it to repair... Any suggestions why that happens?