I have a '91 Rocky with the HD-E engine, and I've been fighting with an overheating problem. In the back of my mind I feel that this is fairly major, possibly a head gasket, but a new problem has come up. Now, when I push the throttle all the way down, and it has to be in gear it can't just be idling, I hear a very loud hissing, almost a screach when the engine is tach'ing around 4000rpm. It seems to be coming from the right-rear part of the car. I know this isn't the exhaust, it comes out the other side. I did just take out that plastic box behind the right rear inside panel to fix a crack in the vacuum check valve that's back there. Could this be coming from that, or could this be coming from the engine compartment and it just sounds like it's coming from behind me?? If anybody has any advice, I would very much appreciate it.