Deceleration clicking noise

tails · Postby **tails** » Wed Dec 23, 2015 9:44 am

Everyone loves a good mysterious sound investigation, right?

I've recently noticed a clicking sound coming from the engine bay of my '92 during high RPM deceleration in second or third gear. It sounds like something clipping something while spinning (like a fan hitting something once per rotation, so I did check the fans). It's a dull 'plastic' sounding click and it matches the RPM but goes away once the RPMs drop below 3,000ish.

Any ideas? I'm fairly certain it's not just lifter noise (the frequency doesn't seem high enough to be lifters), but happy to be proven wrong.

StanTheMan · Postby **StanTheMan** » Wed Dec 23, 2015 10:47 am

does it also happen when you are standing still & juts rev it up?

if it does, it might help you isolate to where its coming from.

If it doesn't....it might suggest the drive-line somewhere in the front.....

I had a resonating sound but not the front while decelerating.at around 3000 rpm. I thought it might have been a baffle inside the muffler or something .
It turned out to be a broken bolt in the exhaust joint.

I'm not suggesting this is your problem. juts giving an example.

tails · Postby **tails** » Wed Dec 23, 2015 11:24 am

It doesn't happen while stationary.

It's quite sporadic, it doesn't even happen every time I drive the thing. The last time I noticed it was a few weeks ago but then it happened again on the way to work this morning.

93_Clubman · Postby **93_Clubman** » Wed Dec 23, 2015 11:35 am

Similarly, the securing tabs on the exhaust header heat shield break & a acceleration or deceleration vibration can be heard around 3k rpm - usually a zinging sound. Securing tabs can also break on bullet muffler near the diff causing a vibration - again usually a zinging sound. And of course, it being intermittent makes it more difficult to identify.

tails · Postby **tails** » Wed Dec 23, 2015 12:00 pm

93_Clubman wrote:Similarly, the securing tabs on the exhaust header heat shield break & a acceleration or deceleration vibration can be heard around 3k rpm - usually a zinging sound. Securing tabs can also break on bullet muffler near the diff causing a vibration - again usually a zinging sound. And of course, it being intermittent makes it more difficult to identify.

I currently have no heat shield header and it's definitely coming from the front of the car. It's a dull and faint knocking like sound, not metallic at all. Imagine someone hitting a small wooden stick against something about once every half a second.

I can't feel it in the steering or pedal either, just a sound.

bruce · Postby **bruce** » Wed Dec 23, 2015 2:56 pm

There are more heatshields further down the exhaust system, before the cat. NA6s were notorious for it.
If not that, good luck, as it could be anything !

93_Clubman · Postby **93_Clubman** » Wed Dec 23, 2015 3:30 pm

Has the noise only started since you fitted the ASI 42mm rad the weekend before last?
viewtopic.php?f=29&t=70270&p=867224#p867224
Looks like you might be located in ADL - maybe one of the forum nearby locals can have a listen if it becomes other than intermittent.

tails · Postby **tails** » Wed Dec 23, 2015 6:44 pm

To be honest, I can't remember.

It may be the ill fitting radiator bumping against the sway bar or something, but it doesn't explain why it only happens on decel and not when going over bumps. It's also not the fans as it happened this morning while the car was still warming up so the fan wasn't running.

speed · Postby **speed** » Wed Dec 23, 2015 9:29 pm

A dull faint knocking sound that repeats every half second. Also described as a clicking sound that matches RPM.

Can you feel vibration that matches the timing of the sound?

tails · Postby **tails** » Thu Dec 24, 2015 12:06 pm

speed wrote:A dull faint knocking sound that repeats every half second. Also described as a clicking sound that matches RPM.

Can you feel vibration that matches the timing of the sound?

Sorry I wasn't clear, it's hard to describe. It does follow the RPM, but not at the same frequency (ie. the knock isn't 3000 knocks per minute), it's just speeding up and slowing down with the revs. Once every half a second was to give you an idea as to how fast it is actually going, and to make it clear that it isn't a buzz or vibration.

I can't feel it through the wheel or pedals.

This is probably all too hard. I was hoping there was something common with MX-5s that made this particular noise, and someone could of just pointed it out, but that's obviously not the case so I'll have to investigate more.

speed · Postby **speed** » Thu Dec 24, 2015 12:46 pm

Can you feel it in the gear stick?

speed · Postby **speed** » Thu Dec 24, 2015 5:25 pm

Could be anything. Usually these things are heat shields. Can also get noises from brake shields. I'd check everything is tight and that you have clearance re the new rad and go from there.
If you suspect something is touching, put something soft in between and test again to see if he noise goes away. Apply common sense with what you put where.
Is a pitty you can't hear it when stationary with bonnet up as that would be easier.
Best of luck