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hissing sound



This may be hard to understand but i'll try to explain......My 77 vette L48 a/t makes a hissing sound somewhere in the engine when it shifts from 1st to 2nd and again when coming out of passing gear. It sounds like air is being sucked in (or out). It is a very brief hiss but noticeable. Could it be a vacuum hose leak? I'm baffled...:confused
Is your air cleaner stock?

Is it possible you are hearing the clutch fan spooling up for a split second?

Any mods on the car?
69MyWay.....Thanks for the response....the air cleaner is stock, the filter is K&N....no other mods....how do I check the clutch fan "spooling up"?
Did this sound just start all of a sudden?

The K&N filter charger will make some more hissing than normal.

The fan will make more of a whoooshing roaring sound. In cold weather the fan clutch may not ever engage. That means that the fan blades are free to spin independent of the speed of the water pump pulley.

So, if the fan clutch is slipping/bad, or if it is just too cool to engage, is may be spinning faster than the pulley for a split second when you bog the motor down for a second during shifting.

To see if it is working right, go out to the car and give it a feel with the engine off when the engine is cold. Grab the fan blade and move it back and forth to see how much tension is behind it.

Crank it up and warm it up to temp. Sometimes you can hear a difference in pitch of the engine when the fan clutch engages (that is the finned thing aluminum thing you see in the center of the fan blade housing) at temp.

Shut it down, then grab the fan blade. You should have significatn resistance on it now as if it is a solid part of the water pump pulley.

Like I asked, is this condition new? If not, you may just be hearing the standard cooling fan noises as it goes up and down with the speed of the engine.
I've been hearing this for some time when it comes out of passing gear. Only recently when it shifts from first to second. This is an automatic transmission.

Nice to see you posting again. How's the 77 been running since you got her back out of the shop? Hissing c/b coming from the carb through the K&N. Gotta get over the WWbridge and come see you guys one weekend. Let us know if you track this down.

......... Nut
Hey Nut, She's really purring nice now and the body shop did a nice job. It's just this little hiss I get just as it shifts from 1st to 2nd. It's probably nothing and I'm probably the only one who hears it, but it will drive me nuts until I figure out what it is. Take care, Hoss:beer
Bobchad.....I'll get the digital camera out today and see if I can figure this thing out to get a picture posted....Hoss

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