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Ok, so I'm still in the process of selling my MX-6. I swear that car waited for me to try and sell it, to start having all the problems in the book, and then some...

Anyway, once I finally replaced the rear friction gear spring, the loud tapping noise disappeared, but now it has been replaced by a different noise.
This one is also a tapping noise, but instead of sounding like metal to metal, it sounds kinda muffled, like something is tapping on a plastic bucket. And then, it starts getting progressively louder for a few minutes, until it completely stops. Then the engine keeps running very smoothly for a minute or so, and then the cycle starts again.

Can anybody tell me what can be causing that?

TIA.

Nobody?

video?

Hmmm...I have no camera, and my cell phone can't record video...

I'll see when my neighbor comes back from work.

When was the last time the cam belt and tensioner were replaced?

Could be a bit of belt slap due to the tensioner being faulty. Normally shuts up though once warm as the oil in the tensioner expands with the heat and sorts it self out. Only other thing I can think of with a similar noise. There's a video on here some where of it. Sounds alot like a muffled HLA.

This is a tough problem to diagnose – especially from afar. I assume this is noise at idle and it is continuously cycling if you allow the car to sit at idle. I also assume the sound changes with changes in rpm – so if idle rpm cycles up and down it could be the reason the noise comes and goes.
I know you recently changed the timing belt and tensioner so don’t think that is likely the source of the sound. Do you have a mechanics stethoscope? They’re cheap and really helpful in identifying the source or narrowing the location of noises. A long screwdriver with the handle pressed to your ear will also work. A cyclical thumping or knocking that disappears and then returns could be caused by one of the belt driven accessories. AC compressor seems like a suspect since it will cycle on and off. Pay attention to the AC pulley clutch since it will engage with defrost selected - it does not have to be switched to AC. I also wondered about the timing belt cover rubbing, but that doesn’t fit your description that the noise disappears and then returns.

Hmmm... The thing is that the RPMs don't vary at all. I will dust off the stethoscope tomorrow, but, so far, I'm inclined to agree with Mike (I did replace the timing belt, but rebuilt the tensioner, so I'm thinking maybe the 30 weight silicone fluid I used was too thin), which is very bad news, because I'm in no shape to buy a new one, and I'm also NOT looking forward to disassemble that part of the engine again...

But we'll see. I'm gonna see what the stethoscope tells me tomorrow...