Engine noise, what could it be?

Question

When i drive my car on occassion it makes a hissing sound, upon opening the hood, it appears to not be coming from any pulley but rather from the engine block. It mostly occurs when i accelerate, getting louder just before a gear change. Its a 2001 Holden Commodore (similar to the Pontiac GTO) Its not a hugely loud hiss, but can be heard from inside the car - thanks in advance

Answer 1

not sure if your problem is the same as i had, i had a similar noise when i was letting the engine brake rather than using my brakes (almost freewheeling but in gear) and that turned out to be a bearing in my drive shaft (new drive shaft)

if its not that it sounds like it could be a bearing in the gearbox how ever if it does it when the car is still could be a bearing in the clutch? clutch release bearing... something along those lines i think let a mechanic have i ride in with you or show him it when its happening he'll tell you more

Answer 2

air leak or vacuum leak

Answer 3

most likely a manifold gasket.

Answer 4

gasket

Answer 5

Its called one of the many flaws in the internal combustion engine, they all make noise.  However on a more personal note sorry dude but that hissing noise means youve blown a seal.  You might be able to reseal it and get everything shaped right again,  but sell it to some rip off in the mafia and go get a car from the police auction they stole and left on the street.

Answer 6

There may possibly be a crack in the PCV Valve or in the hose that connects it to the engine. these hoses become brittle over time as they heat up and cool down. That would be the first place I would look.  

Answer 7

Probably a vacuum leak or something. Check everything intake related for leaks.

Answer 8

a broken manifold seal maybe  - hard to say really.

Answer 9

Sounds like a vacuum leak, but I would have thought you would have noticed it in rougher running.