2nd gear knocks out into 4rd (99 e430)
 

my 99 e430 started to knock out of 2nd gear this weekend.

full throttle, there is a very loud grinding noise then knocks out of gear to third (strong enough to shake the car).

on normal throttle, the cars seems completely fine.

i'm taking the car in to the shop this week, but would like to walk in with at least a bit of knowledge.

any help will be appreciated.