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I did a little more diagnosis this evening with the stethoscope and yanked the accessory belt off.

The noise isn't an accessory for sure since it's still there with the belt.

With the stethoscope it was clear it's not the bottom end of the motor and placing it on the center of the valve cover is fairly quiet too.

However, once I put it on the raised section of the valve cover over the cam gears the noise was easily audible. So i'm guessing there is something going on with the cam gears or the timing belt and/or tensioner/idler.

I didn't replace either since I knew the belt was fairly new and looked in good shape, and feeling the tensioner and idler felt fine when putting it together.

Looks like I need to yank off the front covers and valve cover and crank it up to see if I can isolate it further.

The noise is from the front of the motor for sure too, I placed the probe of my stethoscope on the block near the top as close to where I thought I could hear it and it was nice and silent, so I think that easily rules out any detonation.

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2017 Autocross Chief of Tech

2000 E Street Miata
2015 Focus SE EcoBoost 6 speed
2015 Fiesta ST (Wife's ride)
2012 Yamaha FZ1 sorta crotch rocket? All I know is 150HP and 487 lbs is fun!

Some more googlefu and looks like it could just be a loose timing belt. Apparently you're supposed to spin the motor over by hand one rotation before tightening down the tensioner bolt. I did not do that.

_________________
2017 Autocross Chief of Tech

2000 E Street Miata
2015 Focus SE EcoBoost 6 speed
2015 Fiesta ST (Wife's ride)
2012 Yamaha FZ1 sorta crotch rocket? All I know is 150HP and 487 lbs is fun!

Problem found a remedied. The timing belt was crazy loose.

Not sure how I managed that but there was a lot of slack and somehow it managed to not jump teeth at all, it was still perfectly timed. There are now some semi shiny spots on the underside of the valve cover between the cams and some spots on the plastic covers.

_________________
2017 Autocross Chief of Tech

2000 E Street Miata
2015 Focus SE EcoBoost 6 speed
2015 Fiesta ST (Wife's ride)
2012 Yamaha FZ1 sorta crotch rocket? All I know is 150HP and 487 lbs is fun!

Off the top of my head, I think you rotate it about 1 7/8 of a turn. There is a second mark on the OP housing to the left of the TDC mark. As I remember, you line the mark on the crank with that second mark, then tighten the tensioner. The tensioner springs can also wear out and new belts have more rigidity than old ones. With new belts and old springs I would give the tensioner a little extra push while snugging the bolt down.

Glad you got it fixed.

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92 Miata 1.9L naturally aspirated. SSM 21

Yeah, it's 1 5/6 turns, I had a digital copy of the FSM I followed when I re-did it.

This was a used belt but wasn't the original, it had about 20k miles on it. Tensioner spring had about 110k I assume unless it had been replaced at some point. I did give it a little extra push though while snugging the bolt down. I also checked the tension by hand in all the places slack could have been to make sure there was none this time.

_________________
2017 Autocross Chief of Tech

2000 E Street Miata
2015 Focus SE EcoBoost 6 speed
2015 Fiesta ST (Wife's ride)
2012 Yamaha FZ1 sorta crotch rocket? All I know is 150HP and 487 lbs is fun!