I noticed recently that whenever I hit north of 6k rpms, there's a hissing noise from the engine bay. Any ideas as to what it could be? I have a small exhaust leak at the manifold, but that wouldn't necessarily be it, would it?

 

We need info on the car, like model, year, mods. It could be your intake or the air pump if you still have that.

 

Turbo or N/A?

 

Sorry, I was really tired and totally forgot about it. It's an S4 N/A, bone stock

 

I would start the car and start spraying starter fluid around your intake hose and all vac lines.Sounds like it could be a vacum leak,like when you get a short ram intake you can hear it sucking in the air.

 

Open hood. Check for one Cobra. If snake is present, play flute to calm snake, then remove. If not, continue as below.

Its NOT going to be a vacuum leak. Vacuum leaks only occur.... under vacuum. At WOT (assuming WOT at 6000 rpm), manifold pressure is equal to atmospheric: there is no manifold vacuum. No manifold vacuum, no vacuum leak. Think about it: is any air going to suck through a tiny crack in a gasket or vacuum hose when there is a 3" free-flowing hose attacked to the same system (air intake)?

Since you have mentioned there is an exhaust leak, that is most likely the culprit. Backpressure created in the stock exhaust system, especially the catalytic convertors, will manifest itself as a hissing from the engine compartment as exhaust flow increases and more and more escapes the exhaust system. If you'd like to make sure of this, have a friend shove two potatoes in the tailpipes briefly, while you rev up the engine by hand in the engine bay. The leak will hiss like mad if thats the case. Don't leave the potatoes in there for more than necessary. The rotary makes not for a tasty baked potatoe.
I have witnessed this many times, one to the point where the catalytic convertor melted shut, and caused an exhaust leak at the flanges. The exhaust sounded like 468 angry geese.

So... fix exhaust leak.

 

If not the above, I think it's the air pump air being dumped by the ACV throught the relief passage, especially if you don't have the relief air silencer hose. Note that this sound is not too loud, and it will do it if you rev lower (i.e. 4000RPM's) and release throttle. Try it from the outside.

 

the dreaded cobra snakes must be in the engine bay. thats gotta be it. lol
as taken directly from wikipedia its self.(disclaimer: no attempt at altering content herein have been made for sake of self entertainment).

 

but i digress
yea like scathcart mentioned check that exhaust leak pronto

 

Thanks ********!!! I'm sitting in chemistry lecture and literally just laughed out loud... Where does someone come up with something like that jeeze!

 

Lo and behold, the hissing noise disappeared today. Maybe it was just PMSing or something, because my small oil leak seems to be gone too....

 

Is there a possobility the crankcase isnt being ventilatted and air/oil are escaping? If so i would check your oil cap/oil dipstick, if either of those have come loose i would re tighten them. If the symptoms return id be looking at your crankcase ventilation system, which i cant think of the name of for the life of me.

 

its called the positive crankcase ventilation system (pcv) but close enough.
for air to be escaping fast enough to create a hissing sound might be a bad thing.
i dont "think" the blowby should be that excessive and if it was it would mean a blown side seal. well at least i feel sure about that.

 

Please, not the whole ***....that is just too much!

*** OVERLOAD!

 

 

Alright well i was prett close. Did you check your oil cap and dipstick yet?