Hissing sound


Asked by Sep 15, 2017 at 04:44 PM about the 1981 Chevrolet C/K 10

Question type: Maintenance & Repair

I inherited my dad's 1981 Chevy Silverado C10 and
i noticed a hissing sound coming from underneath.
It sounds really close to the transmission pan. It
has a 350 tranny. It leaks tranny oil too. Could low
tranny oil cause this noise?

14 Answers


Hissing is usually associated with a vacuum leak.


Thank you for your response. I know the engine has vacuum lines but does the tranny have some too?


There may be a vacuum modulator on the transmission.

1 people found this helpful.

You know what i noticed that the modulator had no line connected to it. I will look for that line. Thank u very much for help.

2 people found this helpful.

Yes I believe yours has the Vacuum Modulator. It should go the port on back of carb or intake manifold with or near brake booster port or a source on the intake manifold


The hissing if the hose is gone will be coming from the back firewall side of intake or carb.


Pick up a length of 3/16" steel brake tubing and bend it to fit up to intake and over to the modualtor and cut off the flares. Use a short piece of rubber vacuum hose to connect each end.


Full vacuum from engine intake is needed most often it is located just behind the carb, depends on other factors like stock parts or not but any full source vacuum will be fine.


Some after-market carbs have them in front.




Thank u all. I will check tomorrow morning. \,,/


So i checked the modulator and it was that vacuum line. The only thing is that the hose slips off a little and some air still comes out. Do i need a new hose or can i just add a hose clamp to the modulator end?


I would think a new hose.


Ok thank u very much

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