I would suggest you start off by looking, with a flashlight, down the barrel of the adjuster nut. If the bolt is at the very bottom of the barrel nut then the tensioner's rubber bonding has seperated. the rubber bonding is what the tensioner uses for a spring, so long as the rubber stays attached to both inner and outer housings. Also remember that the threads of the bolt and nut are left handed.
To remove the tensioner:
1)Remove fan blade assembly and fan shroud.
2)Break loose the 3 bolts attaching the water pump pulley and the power steering pump pulley and the 4 bolts for the fan pulley then take the belt off.Then take the pulleys off.
3)Take loose the belt tensioner shock at the bottom mounting point on the tensioner( some have a nut on the back so look first )
4)you will see the 19mm bolt head for the tensioner goes thru a "Y" shaped bracket. There is an 8mm bolt(13mm head) at each points of the bracket.The one to your left at the fan pulley bracket is a special shouldered bolt. The bottom of the two bolts to your right in th power steering pump has a nut on the back of it. Then take the 19mm bolt out of the center of the tensioner and work the tensioner assembly out.
Pay attenion to the way the adjuster bolt is postioned on the rear of the tensioner for re-assembly.
5)re-assemble in reverse order. you will find a white plastic pointer between the tensioner and the bracket, put it on the new tensioner assembly to the right hand side of the marks(as it tensions the pointer will move to the left).
As for the tensioner shock check the bushings at both ends to see if they are loose in the housing. Try to compress/extend the shock you should feel resistance both ways.
If you don't feel confident in your ability to do this job( if it's needed ) then seek out a reputable MB shop to do the work for you.
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop