somebody\'s knockin, should I let him in? I had that familiar little hampster knock in the engine compartment that is normally associated with an exhaust leak. So, I replaced the header gasket on the passenger side. A little while later, I noticed it was knocking again, so this time I bought some copper gaskets and red rtv. Deciding to not be a lazy a$$ again, I did both sides, thread-locked the bolts and put those nifty header lock washers on there, checked the header flange for any warping, and bolted it all down snug. Waited about 6 hours and drove it. To my complete and utter dismay, it was still knocking. I was about to take the sawzall to the headers when I decided to learn from my past mistakes (see cdrom destruction to find a bad ide cable) and listened for the knocking. It wasn't the exhaust at all, but rather the water return from the heater to the block. It was like it was sucking or something, causing a knocking sound. It vibrated the return hose when it would "suck". So I thought, heck, I just added antifreeze, but let's check again. I opened it up and there was plenty of antifreeze. I thought I would top it off to be safe, and added another 1/3 of a gallon. So, now that I have babbled on quite long enough to paint the picture, what the heck is causing the knocking? Is there some sort of backup in my heater, 20+ year of crap in there causing a pressure backup maybe? Is this going to damage my engine? What say ye?