I'm about 4-5 days into my new heater core and I don't hear the noise anymore (again...knock on wood), so only time will tell.

I'm guessing I might have had a slight blockage and the hissing noise was faster moving coolant liquid rushing past the blockage upon acceleration. Because obviously the water pump rotates faster when you accelerate so fluid moves quicker. Again, I just don't know for sure, cuz my heat always worked great and my coolant is bright orange from regular servicing, so my instinct tells me a blockage was highly unlikely.

But the noise seemed really sporadic to begin with. Some days it was clock work - 3,000 RMPs and higher on every acceleration. Other days it was faint and I couldn't hear it until 4,000 RMPs (obviously a hard acceleration). I even bought a engine stethoscope and was listening to different pulleys because I thought, for sure, it was a pulley.

I think I cracked my heater core because I had AC running to both upper and lower vents, let me explain. Normally, I just run AC to upper vents on a daily basis in summer. I went on a 6 hour road trip, so for the sake of comfort, I had AC running to both the whole time. Well my GP (idk about everybody else's) doesn't not have a Heater Control Valve, so AC is always flowing through the Heater Core even in the summer. A heater control valve will allow the cooling system to bypass the HC when car heat is not needed (aka summer). But for in my case, if the AC is on, air is never blown over this HC (to produce warm air), so you never know the difference, although the floor air vents passes right next to the HC cover. So I speculate that the constant cold air next to my HC for such a long period of time gave it a stress fracture. If you've never seen a stress fracture, put boiling water in a glass cup, pour it out and then put in cold water, it will crack the cup in half.
Or maybe I'm over analyzing it and the HC was just old and started leaking.

Anyway, there's a really nice thread on here for heater cores I used: http://www.grandprixforums.net/threa...61#post1444261
It wasn't the most difficult job, more so challenging because of the location. I definitely wouldn't want to do it again, my back hurt for a few days.