T5- Z spec 5th gear problem

Hi, I have installed a brand new T5-Z spec in my Plus 8 Morgan (240 hp). Everything was Ok and quiet for the first 75 miles. When I took it out the next time as soon as I put it in 5th gear and put it under load, this loud rattling noise came from the gearbox. The other four gears are quiet. The shifter also jerks back (1/8" at the top of the 6" lever) quite violently but a few seconds after the noise starts. The noise is a loud whirring and sounds like someone gargling. Oil level is correct and ATF. The box is out of the car and all looks ok inside. Any suggestions as to the cause please?

jrichker said:

The snap ring on the countershaft may have popped loose...

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Thanks for your reply. I'm in the UK so expect a big time difference in the responses.

I thought it might be the snap ring but I checked that and it is still in place. Everything looks ok both inside the tail housing and the box itself. The 5th gear assembly seems very sloppy though and something on there must be kicking back to make the shift lever jerk back (as described in my post above).

With the syncro assembly engaged in 5th, I cannot understand what is causing the loud whirring noise.