hey everyone im mainly posting this for my dad, he has a 97 s10 blazer and hes been getting this loud obnoxious knock from under his dash (right under where the radio and fan/climate controls are) and this only happens when the truck gets up to operating temp. and when accelerating from either a complete stop or while moving to gain speed. now he thinks its the heater core and i dont. but going off it being the heater core we've tried all the fixes...using a radiator machine to suck out all the fluid and the possibility of there being an air bubble, and just today we tried running water straight through the heater core and and it still does it. does anyone have any ideas on what it could be?

 

This may be nuts, but next time he goes out go with him. That spot is also directly above the bell housing. Might want to make sure it is actually coming from the dash and not the console. How do you like sitting on the floor?