At its loudest when I pull to a stop and the car is idling. After 20 seconds of this sound at idle it usually goes away. Revving the motor gives mixed results. Sometimes the noise rises with the rpms and then goes away. Other times it goes away almost immediately when the rpms start to rise.

I thought the oil level might be low and I was starving the motor but oil level is fine. Of course that doesn’t mean that the oil pump is pumping volume but at least the level is fine.

Only things that I’ve changed or adjusted before the noise started were:

1. Changed to a different AFM. Thought this might be detonation but the mixture screw is turned all the way in. So if anything it should be running rich with the mixture screw all the way in.

2. Timing was checked and adjusted. This noise is present at 8 deg. BTDC (stock setting) and 4 deg. BTDC.

3. Added heating ‘j’ tubes, flapper boxes and the rest of the associated hardware to the drivers side. Incidentally it *almost* sounds like the noise is coming from the drivers side. I thought it could possibly be the small flap that cuts off air going to the heat system at idle. But it just sounds too loud for that and even if it was that flap would really have to be snapping back and forth a bunch.

https://youtu.be/ELe26KeljpU

Tough to tell from the clip, it sounds like a noisy valve or maybe an un-pumped hydraulic lifter. Do you have hydraulic lifters? Strange that it’s intermittent.

My money is on the bad lifter hypothesis.

Click to view attachment

Agree with all of this. Sounds like a hydro lifter that’s not pumped up, that is, until it starts making the sound intermittently and at a different speed than the engine rpms, but maybe that’s just the recording. If it’s a lifter, the noise will speed up/slow down in conjunction with the engine rpms.

Make sure your valves are adjusted correctly. Gap is different for hydraulic lifters (if that’s what you have).

Want me to come listen to it?

Clay

1.Valve adjustment
2.Alternator bearing
My bet is one

My car would do intermittent noise similar to yours and would go away but come back. Bad valve seat was the culprit. Sorry about the bad guess but just sharing my exp.
gg

I appreciate all the replies. I thought valve adjustment, too. Initially I thought odds of it coming from the valve train was low. My reasoning was that a valve adjustment was done not that long ago. Second was that the sound just kind of came out of nowhere. I figured valve train would start low and over time get louder. This was more not there one day and then there the next.

Absolutely would welcome your expert ear.

Alternator belt? Let's hope....
gg

So do you have hydro lifters?

Lets hope not.....

Negative on the lifters.

Typical of a loose valve seat. Hasn't completely dropped yet, but sliding in and out of it's recess.

Have you had the HE's off?

That's my guess, too, having suffered dropped valve seat(s) on a 911 engine.

It lately. Maybe a few years ago.

I’m leaning towards this. I’ll have some time to get under the car in the next days.