The GM quick disconnect heater hose connection. What you had is not rare and it's a ticking time bomb every GMT-400 owner gets to experience the joy of diffusing.
I noticed mine because it was leaking; it had a nice crusty build-up of dried DexCool in and around the connector part. Once I educated myself as to what I was facing with replacing it, I had to wonder what design engineer would spec something like this in the first place? And then make it out of cast pot metal that, essentially, dissolves over time. I was lucky as mine came out in one piece; but I was ready for disaster. I also read the special tool used to remove any remains in the manifold is often useless and can make a bad situation worse; the remaining pot metal is so soft and crumbly the tool doesn't grip and simply grinds away at whatever's left in the manifold ...which of course dumps all the grindings into the manifold (you have to coat it with grease so any metal "crumbs" stick to the tool). So I opted not to get the tool and tried my technique. I soaked the thing with PBlaster and let it stew overnight. Then I used a deep socket installed on my DeWalt cordless impact driver (what's normally used to drive screws into wood). The PBlaster treatment and the relatively gentle impacts of the driver worked like a charm and it all came out ...what was left of it. A sacrificial beer was offered to the engine gods and Lord Murphy. I replaced the OEM quick connector with a brass barbed and threaded hose nipple scored at Lowes (plumbing section). I used brass because brass and cast iron play nice together; no weird electrolysis or corrosion issues to deal with down the road. I used high pressure/high temp pipe joint compound on the manifold/nipple joint. I trimmed off the connector flange on the metal heater hose, slipped on a short piece of rubber heater hose and double hose clamped it. A single hose clamp on the barbed nipple was all that was needed. I let it sit overnight to allow the pipe joint compound to set up. The new connection works like a champ and it's easy to keep an eye on the short connector hose. I've had this in place for about 3 - 4 years without issue.
Now, the restriction may have something to do with getting better coolant distribution throughout the water jacket in the engine instead of allowing more flow through the heater core, IDK.
