I don't know what howls means. I assume it makes a loud noise all by itself. Is the noise a loud hum? Hum being the sort of noise you get if you touch the end of your guitar cord. Of is it more of a WOOOOO sound? Or is it a high pitched SCREEEEECH? It is capable of making noises, but the nature of the noise suggests different problems.
If hum is the problem, then we look for either DC on the output jack or we look for loss of filter cap function. And that latter would be either a cracked soler connection to a main filter cap, or the cap itself has failed.