1998 Volkswagen GTI Catalyic Convertor problem

Hello,

Thanks for you time, took my car for an M.O.T Passed with flying colours, next day my CAT started to rattlr, I knew then the emissions test might if blown my CAT that might have been on its last legs.

Took it back, they spent all day trying to source a CAT (for a non Turbo 20V AGN Engine)
Thet apparantly tried fittinh 5 CATS!
I referred them to GSF which delivered them the correct CAT the next day, the fitted it in 30-60 minutes.
I drove a couple of 100 feet and my exhaust (I think) is making a horrid deep vibration sound that is felt t through pedals and steering wheel and seats (also my steering felt a bit strange like it wasnt balanced properly)

It happens in all gears but more prominent in 1st, 2nd and 3rd between 1500-3500 RPM (ish).

I drove straight back and took a mechanic with me for a drive and he noticed it straight away even though they took it for a road test?

They were dumbfownded. On the ramps and in nuetral the car does not show the charatiristics above when revving, they perfomed the hand brake enginge mount test and showed me that the engine mounts could be loose and the exhoust down pipe (flexi part) is hitting the fram of the car, reccomending changing mounts but they admit not knowing the problem.

Before the M.O.T I never had any issue inthe 4 years I owned the car (I service it every 6000 Miles sparks oil air etc.)