How many miles on the car? Does it still have the single row timing chain? (Some have been converted aftermarket to dual row).

The 380SL should have the timing chain inspected, if not replaced, every 50-60,000 miles. The replacement/service should include upper rail inspection or replacement and tensioner replacement. My 420 SEL tensioner failed and ate up half the engine. The chains stretch over the useful life and will become more prone to skipping sprockets and running your pistons into your valves....not a good thing.

The noise you describe is possible broken rail or slack in the chain....both will dramatically increase chances of big problems.

Chains don't always have to be replaced - it is a good idea, but the chains rarely break - the tensioners and guides fail before the chain.