Help diagnosing engine noise

[texdave]

Video of engine bay

Details: Stock QJ, stock intake, stock cam, headers, no EVAP equipment (factory), 18-18.5 inches of mercury vacuum at idle

I can’t figure out if the droning/burbling/hissing noise I’m hearing is normal or not. I thought it was a vacuum leak but all the lines in the bay have been replaced or checked out as OK. It seems to originate at or just behind the carb. It is also louder when engine vacuum is higher (idle or coasting).

I recently rebuilt the QJ using Cliff’s performance parts. Ended up with slightly better MPGs and slightly higher vacuum at idle. Could there be a leak on the throttle body or where the carb meets the intake?

Only other place I can think of is the HVAC control unit but the nose is louder in the engine bay than inside so I doubt it.

Any other place I should look at?

 

[PrairieDrifter]

The brake booster line. It should be a steel tube that goes to rubber at the booster, and is threaded into the back of the carb base usually.

Also either the transmission or hvac controls have a port behind there too, on top of the intake.

New carb gasket? I like the thick ones.

 

[texdave]

@PrairieDrifter I snugged up the brake booster connection at the back of the carb when I put the QJ back on the intake but I'll feel around it just in case there's something off there. However, I didn't check the other end of the booster line so that's a good call.

There's a vacuum tee off of the intake manifold that feeds both the cruise control and the HVAC and both those lines are pulling good vacuum. I guess it doesn't hurt to bring out the vacuum pump tester and triple check those lines as well.

And yes, I have a 1/2" gasket under the QJ where it meets the intake.

 

[texdave]

Today I pulled out my trusty smoke machine and connected it to the vacuum booster vacuum line. No leaks from the carb or intake but there was a real nice big smoke trail coming out one side of the manifold vacuum tee.

Earlier in the week I had a shop install headers and they must have disconnected the vacuum line that goes from the tee to the HVAC vacuum reservoir and forgotten to connect it again.

Simple enough to connect it back up. After that, I did a quick check and adjustment of the mixture valves on the carb with my vacuum pump and the rig is back to running really well.

Lesson learned -- check and double check the easy stuff.