After pouring seafoam into my brake booster line, and the epic smoke signals, the engine will hiss. i'm pretty sure it's not any lines as most of all the tubes have been replaced but here's what happens...

after a high rev and letting go of the throttle, there will be a hiss or slight grinding sound coming from around the intake manifold, it almost sounds like a turbo engine sounds. it only happens when you drop revs from about 4k to 1k or like high to low, it doesn't hiss when going from low to high....any thoughts?

 

Man.... I personally never dump that shiz right into the intake manifold (doesn't evenly disperse). I run a bottle through w/ a quarter tank of gas.

 

well, it worked so far but that hissing has really made me think about regretting doing it...i really need to clean the valves/throttle etc it's got 174K and i'm worried...