i have a 98 f150 4.6. my truck has been making a noise from the dash it seems like. sounds like water is boiling. my truck never overheats, im not losing any coolant. my heat works great in the truck. the noise is just loud and annoying. its been doing this for a while. i recently put on a ac bypass pulley cause the compressor locked up and it went away until today, and it started again.

and by the way, it doesnt make this noise or any noise at all after i shut the truck off. and it starts as soon as a start the truck on a cold or warm start.

any help? ideas?

 

Probably air in the cooling system.

Check for any minor leaks in the system, especially near or on the core.