Probably just the results of the clutch disk. You'll notice in other manual cars a slight clunk when on and off the gas. This is probably just more noticable with a 6puck clutch than a stock one?

If I may interject...
I think the inportant thing to understand in regards to this subject is that these these forementioned driveline failures are the result of interrelated material limitations. The OEM motor mounts were not built to handle the amount of torque which is being delived buy these high HP applications. The design of the bonded 93-early 94 driver's side, aluminum arm/ bonded motor mount is extremely poor in my opinion and fails frequently. The later 2-bolt/liquid-filled puck design fails less, but still falls short because the comfort-focused design allows far too much power plant roll. As has been pointed out, this rolling motion combined with the ring gear forces test the limitations of the PPF when it attempts to transfer rotational force to the diff, which kept more-or-less torsionally static thanks to the diff mount arms. With that being said, stiffening the PPF - if it doesn't break first - is not in itself a solution. It will only make for a more efficient application of tortional force to the front end of the differential casing. The heart of the issue is that the rear most end of the differential is being helf firmly in place via the diffmount while the front end of the differential is left hanging subject to torsional forces which originate at the engine. How could a stiffer PPF possibly solve this problem? it can't. With that being said it makes more sense to only reinforce the power plant frame where absolutely necessary. Reinforce the hell out of the fingers and the pass through section and leave it at that. The fact of the matter is that you want the PPF to be able to flex torsionally while retaining vertical rigidity or you will risk cracking the diff. By increasing torsional rigity you are just increasing the efficiency with which the engine transfers torsional force to the differential. If the engine and diff rotate slightly then so be it, but allow them to do so somewhat independently. An IRS is the only appropriate solution IMO, but a proper setup can be cost prohibitive. The most cost effective and pragmatic option is to upgrade your motor mounts and and diff bushings and reinforce the PPF just enough for it not to break. Stronger is not universally better.

sorry to bump this old thread but im trying to find some pics of the PPF modifications, PLEASE can somone help me out with this!!!!

He welded plates across the openings on the sides from what I remember
It wasn't designed by NASA or anything

Is it just like this (see attached)??

Looks about right

Other people have taken the route of simply spot welding the cross sections to prevent flex (See black attached)

This seems to have been adopted by a company called makers racing also do the same thing in a nice metalic blud (see blue attached)

Any thoughts on this?

Aditionaly i found a random creation where someone had reinforced teh sides (presumably where they tend to start to crack) in a nice gold paint (see gold attached)

Yep that's my old frame!

Filling in truss with plate like this just adds weight without increasing stiffness. Anyways increasing stiffness without increasing strength of fingers will just make it more crack prone.

What would you sugest then?

brace the diff and trans or reinforce the ears

that's a nice piece ive got one works good

diff brace and gearbix brace fitted, but i want to stiffen this up also as ive got a friend who managed to snap a banzai style diff brace and his ppf at the same time at a drift event....

Curious did you even read the thread or are we just talking out of our ass here on my mod not adding any stiffness. Have you actually done a similar mod to prove otherwise? The main purpose of me adding the plates was to decrease the twist in the ppf that was present before I did the modification. As the engine torqued over to the right, the ppf does the same. I wanted to eliminate that twist which is exactly what the mod did. My throttle response increased and my shifter became rock solid after the mod.