Hopefully someone can help me with this same problem. I did replace the flex plate about three weeks or so ago since it was making a really loud knocking sound. Took it in to my mechanic and long story short, it was my flex plate that was making that sound and it was due to the flex plate being cracked.

Fast forward about a week after my mechanic replaced the cracked flex plate and the knocking (more like a pinging noise) came back again. Brought it back to my mechanic about a week or so ago and after having it for about a day, he advised that it was a bolt that was too long that was hitting my engine and he had to file down the bolt. Again everything was honky dory and no sounds coming from under my vehicle.

Fast forward to Tuesday of last week, the sounds came back after driving it for 2-3 days. Again it's more like a pinging noise, and still coming from underneath my car and from where it sounds like it's coming from, it's around where the transmission and engine connects. I haven't had time to bring it back to my mechanic but maybe you folks can help me out first so I could maybe help my mechanic know where to look or what to look for.

Thanks.

Might be best to post a video or sound file of your sound. This may help narrow it down

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Did your mechanic put loctite on the flex plate bolts (both the inner 8 bolts and the 6 bolts that hold it to the torque converter)? Did he torque them to spec? It's possible that some of the bolts are backing out.

If all of the correct bolts were used when he replaced the flex plate, none of them should have been too long. It's a little confusing too because no bolts should ever be able to hit the engine (again, the 8 inner flex plate bolts go into the crank shaft and the 6 bolts go the opposite direction into the torque converter).

Hope this helps

x2 on a bolt backing out.
I had the same basic issue, but not with my flextplate/torque converter.

I replaced my catalytic converter, and a few months later got a bad knocking/rattling noise when in gear, but especially noticeable when in reverse.
Turns out the bolts that anchor the cat mount to the frame had backed out a bit and the mount was rattling something fierce. The slight angle change when in different gears must have set off the rattling, vs when in park.

Following this thread, sometimes i feel like i hear the same thing coming from my 01 but I mainly notice it when the ac is on and sometimes goes away after turning it off

Check the U joints and drive shaft by grabbing it and seeing if it has any "play". It should have none.

I have this on my 99 and have had it since I bought it 90k and 5 years ago. Mine is a bad pulley on the compressor. I took the AC belt off and the noise goes away entirely. So not dangerous at all, just annoying until you get used to it. Not worth the fight removing the compressor to replace the pulley, discharge and recharge freon, etc. I'll just leave it till it really breaks.

Yeah I feel the same way, no matter what i always say it’s a 20+ year old car and I’ve got 217k miles on mine it’s gonna make noise 😂

I'm reviving this to see if anyone figured out exactly what the issue was?

Was this the noise, guys?

https://www.youtube.com/watch?v=zE6zf1XVX3o&feature=youtu.be

* Noise only occurs in gear once warm

* On the first start up, rarely, the noise does not occur until you goose the gas a bit in gear with your foot on the brake. This starts the noise occurring and it won't go away after it starts.

* My flexplate and bolts looked perfectly fine, bolts were tight.

The problem is not caused by any of the following:

1) The rod bearings (replaced mine, no change).

2) Crankshaft end play (measured mine, well within Toyota spec)

3) The water pump, timing belt, timing belt tensioner, timing belt tensioner pulley, or timing belt idler pulleys (replaced mine, no change).

4) The AC compressor pulley bearing (replaced mine, no change)

5) The V belt tensioner pulley (replaced mine, no change)

6) The V belts (replaced mine, no change)

The plastic engine cooling fan and or the fan clutch?

Once upon a time.....there was a similar noise and it was a motor mount.

It does kind of sound like that at first, or the timing belt hitting the plastic timing cover, but it isn't. The more you listen to it, the more it appears to be a metallic sound.

The fan clutch seems fine and fan bearing has zero play. Stethoscope is very vague and generalized, but it seems to be coming from the front of the engine somewhere. I could be totally wrong though.

you said you got it narrowed down to front of engine, so that eliminates flex plate. It is a bit of a hassle, but take off one belt at a time and see if noise goes away. I had a bad ac idler pulley one time, but that was not a ticking sound, but until I pulled the belts off I was not able to isolate it and my brain was going worse case situations. The idler pulley spins and can make noise even if ac is off and ac clutch is not engaged.
what about harmonic balancer ?
just brainstorming

Mine has developed this noise.

Only happens once the truck is warm, like after driving for a while. A/C on or Off doesnt matter.

It is a definite metal tap/clunk noise that is affected by engine speed. Sometimes it's quite loud, others its not. Only happens at and just above idle, it either goes away or is drowned out at higher RPMs.

Still get same power, mpgs, and no drivability issues.

I had a similar strange metallic knocking that only happened when in gear. It ended up being the exhaust system contacting the transmission. I could not reproduce the sound when sitting in Park; but ended up having my wife hold the 4R in Drive while I listened from the side. Once I replaced the exhaust system the rattle and knocking stopped. Apparently the mount rubbers were broken down allowing for extra movement and the muffler had a large amount of rusted internals allowing for a rattle or knocking depending on RPMs.