My radiator has had a leak for some time now and yesterday morning I was driving to work, and I came to a stop light and when I was braking it sounded like water was sliding/pouring out from my dash. I pulled over and checked everything, and all seemed well and there was still water in the radiator to the cold fill line. So yesterday afternoon I bought a new radiator and installed it. Filled up with 50/50 antifreeze, about 3 1/2 gallons of it. I drove my truck afterwards and still heard the noise, went home parked it for about an hour, topped off the radiator again and drove it and never heard the noise again. BTW, I did have the heater on high when filling with antifreeze.

So here is my problem, now when I drive it there is a pretty loud hissing noise coming from the dash area, and when I turn the heat on it goes away, and comes back when I turn the heat off. Any ideas?

and my heat works great, and I do not have A/C. I bypassed that a long time ago.

98, 4.6, 4x4.