Hi guys,

My friend's 75 280 has always made this strange hissing from the intake: https://youtu.be/mjrzNhHvXL4.  Any ideas what this could be?

We took off the cone filter and when we did we could hear it from the afm, so we took off the afm, and could hear it through the throttlebody, when we took off the throttlebody the sound stopped (kept the engine running by pulling the temp sensor cable and left the tps switch in to run rich enough for it to not die).

We've checked multiple times for vacuum leaks using both a smoker and a can of starter fluid on the manifold and there are no leaks.  We also tried 2 other throttlebodies and both make that noise.  I've never heard if this issue - has anyone encountered it?

 

-chase

Is the PCV system intact?  Might be pulling through the PCV valve on the bottom of the intake manifold.

Just now, Zed Head said:

Is the PCV system intact?  Might be pulling through the PCV valve on the bottom of the intake manifold.

The pcv valve is new but I’ll have him check that

If the system is not intact you can suck a lot of air through the PCV valve.

How did you keep the engine from revving to 8000 RPM with no throttle blade?

6 minutes ago, Zed Head said:

If the system is not intact you can suck a lot of air through the PCV valve.

How did you keep the engine from revving to 8000 RPM with no throttle blade?

Intact in what sense?  The hose is connected to the block and intake manifold with the new pcv screwed into the intake.  Is there anything else I’d be missing?  The valve cover hose isn’t plugged either.

Oddly the car barely ran without the throttlebody on.  I’d have thought the engine would freak out too.

That's what I meant.  You didn't answer so I couldn't tell.

With no throttle body there is effectively no vacuum being pulled on the intake manifold.  So any leaks that would make noise with vacuum will stop making noise.  Could be an injector seal, the gasket around the cold start valve, intake manifold gasket.

I guess you could start spraying areas with carburetor cleaner to see if the RPM's fluctuate. 

Just now, Reptoid Overlords said:

I guess you could start spraying areas with carburetor cleaner to see if the RPM's fluctuate. 

Nevermind I see where you guys already did that

Just now, Reptoid Overlords said:

I guess you could start spraying areas with carburetor cleaner to see if the RPM's fluctuate. 

We actually did that.  Sprayed absolutely everywhere to no avail.  I don’t think there are any leaks because the rpms drop drastically with the oil cap off

I watched the video and don't hear the noise.

Are all of the various vacuum ports on the intake manifold used or sealed?  Charcoal canister, AC system control, brake booster, etc.

I'm wondering if that aftermarket air cleaner has something to do with it..but I run a similar setup and don't have that noise. 

He said they took it off.

30 minutes ago, Zed Head said:

I watched the video and don't hear the noise.

Are all of the various vacuum ports on the intake manifold used or sealed?  Charcoal canister, AC system control, brake booster, etc.

You hear it especially well when I put the camera right next to the air cleaner. It’s a loud sucking noise much louder than my car.  You literally can hear it coming down the street it’s really strange.