I know that the locking system of the tanklid can be heard sometimes and that it needs a little re-adjusting, but noise from the trunk lid...
The vacuum pump for the trunk lid is situated in the back panel at the right side from the lock. Remove the plastic upholstery and try to find out what the problem is...can't be that hard. There is only one pin going from the vacuum pump to the lock which rotates a part inside the lock.

greetingz & good luck

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)