Intake manifold noise

[HSL]

Got this strange chiming noise coming from the intake manifold but it only appears after the engine has reached operating temperature. Its only present at idle or when I get off the throttle, once I give a little throttle the noise disappears. Adjusting the idle screw up or down does not make any difference. It does not affect the performance as far as I can tell. So I been thinking that it is vacuum related but have been struggeling with this now for a while without being able to figure ot out. Have done the egr delete, blocked off all parts of itand blocked off the line going to the throttle chamber. 

I've attached a video, the annoying sound is easy to hear from approx. 20 sec into the video (never mind the rough idle at the end. Got some moisture after washing the car but that does not have anything to do with that sound as it was there before the car was washed.)

 

1977 280z, stock l28.

[siteunseen]

Shot in the dark but possibly the pcv valve under the manifold is chattering somehow.

[Mark Maras]

 Use a stethoscope or my favorite, a 4' piece of 1/2" garden hose, to locate the source.

[dutchzcarguy]

At a certain airspeed something starts to vibrate, maybe it's in the air mass meter.  Take the airfilter and stuff in front of it off and look if it's gone..  For the running of the engine, got an airleak?  it's not running well..

[Zed Head]

All I hear is an engine that seems to be at high RPM, but I'm on a computer with bad speakers.  A "chime" is like the noise a wind chime makes when the wind blows.  Dainty.  Didn't hear that.  What is the idle speed RPM?

 

 

[dutchzcarguy]

3 hours ago, Zed Head said:

I'm on a computer with bad speakers. 

With headphones it is very clear to hear..

[HSL]

The rpm in the video was a bit high, usually the idle is at 8-900 rpm. I'll try to record a new video and see if I can capture the sound a bit better.

 

Maybe chiming is not the correct word to use but its a bit hard to describe the sound... Coming somewhere from the intake but really difficult to locate so will try with some kind of stethoscope.

[dutchzcarguy]

A long screwdriver between the spot you want to hear and your ear, just lay your ear to listen against the top of the handle.. it works..

[superlen]

Start pinching/wiggling vacuum lines and see if you can get the sound to change in pitch/volume. That might lead you in the right direction. The air regulator changes airflow through some largish hoses as the engine comes up to operating temp so maybe something is up in that circuit/system? Also you can spray WD40 or similar around hose connections to see if the pitch changes.

Len

[HSL]

So, after some more checking and listening the source to my trouble is the throttle body or the bcdd. According to FSM the bcdd shall not be adjusted, it should be replaced if it fails. Don't find it easily available so guess I will try to disassemble and clean it. 

 

[siteunseen]

I adjusted mine back to working proper. Seems like ccw brought the rpms down? But I am most likely 180 off.

 

[HSL]

It was the BCDD that caused that annoying sound, blocked it off and then the sound was gone. So I believe those small cracks in the diaphragm is the reason for this. Is it possible to buy the diaphragm anywhere? Have not been able to source it so far...