Another game of 'guess that sound'
 

This is probably nothing, but I'd feel better knowing what it is.

I have noticed a sound that I cannot easily explain. It is consistently reproducible. It happens between 3200-3300 rpm. It is just loud enough that I can hear it but I don't think I could record it. It could just be a resonance in the exhaust. What raised my attention is that it reminds me of what detonation sounds like but I don't think it's consistent with that since it goes away outside this narrow rpm band. And frankly it's probably been 20 years since I've heard that sound in a car.

When I notice it the most is taking my foot off the gas a little. Example: I'm cruising along in 3rd gear and a car ahead starts to slow down a little and I come off throttle or I go over a hill so I let off the gas a little to maintain the same speed. I can also reproduce it by accelerating up to ~3200 rpm and then staying there. Higher or lower rpm and it goes away. Going through that rpm range, there is no sound. Taking the transmission out of gear it goes away which suggests that it is load related. Car pulls strong all the way up to the rev limiter.

My first thought is that it was an exhaust burble and that may be the answer. I have a straight pipe and 1-in/2-out exhaust w/ factory heat exchangers and decel valve is plugged. What doesn't fit is that the sound is more metallic. Valves adjusted last year and there were two of us. We doubled checked everything.

Another thought was maybe knock/detonation but that doesn't make sense. I run 93 octane and just for giggles I put octane booster in at last fill-up with no change. I can double-check timing tomorrow but it was checked last year when the engine went back in. Knock would likely get worse with load and rpms too, right? This does not get worse.

When I had the engine out last year there were no broken head studs.

Like I said, it may just be normal but I'd feel better knowing that before taking off for Amelia Island.

Thoughts?

Another game of 'guess that sound'
 

I am by no means an expert but I do have a "mystery sound" experience. Similar to your RPM range too. My sound turned out to be the distributor just before it ate itself from the inside out. Car was running great and pulling strong up the the point of grenade.

When bad things happen at 3-3.5K
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It's probably not your issue, but you just might want to take a peek at the inserts and drop a tad bit of oil onto the shaft. Unless that has been part of your usual maintenance. If so.... Then these aren't the droids your looking for(wave of hand)

Good luck with your search.

Well that is an interesting idea.

I had the dizzy out last year and oiled the shaft but I have never looked deeper than the reluctors. It does not seem to have a lot of play so I never bothered.

I have thought about having it rebuilt to recurve it and get rid of the vacuum pod. Maybe that is in my future.

Was your sound load related? If you just revved the engine was the sound still there?

93? Octane booster? Your car is meant to use 87.

Maybe a timing tweak would warrant 89. Did you put in P&C's?

Original P&C's. Like I said, the octane booster was just for giggles. It has made no difference.

Replacement cruise cable came in. Grant and I are going to troubleshoot the cruise control this afternoon to see if we can get it working so we'll go for a ride and get another set of ears on it.

Dizzy rebuild and recurve with vac delete is right around a $850-$950 with Barry (flat6recurve.com) in Michigan. I opted to leave the vacuum can on and just plug the ports....didn't see the value in removing the can and permanently modifying my original distributor.


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Thanks, Garrett. I may still look into that.

Well, the mystery seems to have been solved. Apparently, it was the remnants of the old cruise cable because I replaced it while I was waiting for Grant to come over and when we went for a drive it was gone! I guess it was vibrating on something.

Still no criuise though. Seemed there is a problem with the clutch switch. The switch itself seems to work correctly but mechanically it is not switching correctly with the clutch is engaged. Need to do some more digging on that topic.

Tirwin,
Take all of the sound deadening material out of the back seat and shelf area, remove all of the original interior and cover with thin RS felt carpet.

You'll have noises to keep you busy for years :)

Another game of 'guess that sound'
 

Sorry, just saw your question. My sound occurred under load then would back off and subside as I came off the throttle. I literally had a few minutes of mystery sound before I exited the freeway, then all hell broke loose. However, it sounds like you solved your riddle. Congrats!

In my case, diagnostics, search and resolution was greatly assisted by Kurt at PartsKlassic in Scottsdale, AZ. He was instrumental is finding me a two year only dizzy, made of unobtainum. He does all thing dizzy and then some.

He and his wife are really great people and spent a lot of time on the phone with me.

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