Yep, that's where the sound comes from, but the levering forces come from the engine not being stabilized. Without the dogbone fully connected to the firewall, the torque on the engine goes into the motor mounts, into the subframe, which pivots on the tower mounts, flexing the floor up and down. My car has the transmission-to-subframe front member steady which tries to keep the motor mounts from flexing... which translates into more torque on the subframe mounts... and the floor flexed. I fixed the upper dogbone and the floor stopped flexing (no matter how hard I try ).

I bet it is not the dog bone but rather the rear, lower subframe mount. Well, not the mount but where it attaches to the heel section of the floor. More and more, I am seeing cracking in this area and every time one gets on and off the throttle, the metal flexes and the clicking is that metal flapping back and forth. 

Lift the carpets and check, VERY CLOSELY around the mounting point. It is a strap with two captured bolts that attach the rear subframe mounts. 

+1 for the engine steady lower firewall bracket. I had a very similar 'click' or tiny 'snap'. I eventually traced it to a metal fatigue crack on the right-side floor pan. My lower firewall bracket had failed, been brazed back on and failed again. With insufficent support, the dogbone fatigued and broke a hole in the plate that provides the upper bracket. With no real support, the engine twisted on the subframe mounts, levering on the under-floor one and causing a 3 to 4 cm crack in the floor pan.

I'll bet you a beer that the dogbone bushes are shot.

When you look at the firewall engine steady bracket as Mur suggested be sure to look at the bottom/lower part of it as that is where it cracks and shears off. it is not easy to see with the engine steady in place you may need a mirror and a flashlight. Another thing to try is have someone start the car and with it in first gear and the handbrake on release the clutch to see it you have movement there.

Will try both, keep you posted. Thank you

Have a close look at the firewall bracket of the upper engine steady.

why not try it with the engine off, helper operating the gas pedal and you with the hood open looking for the sound.

I hope someone can help me with this strange problem I'm having with my 91 Mini. When I drive my Mini and step on the gas pedal I get a "CLICK" sound from what sounds like it's coming from behind the Speedometer cluster or just behind the dash or in the upper steering column area. It's not a constant clicking but a click when the pedal is pressed like after a shift or on acceleration. Anyone have this problem, is it common with other Mini's?

Thank you

A