I have many chains that have developed a ' tight spot' .... but only after I left them out in the weather for months with no lubrication... a chain inside a protected space like our MB engines should not do that unless they are in DIRE shape.
However, for the record, my impression of the speed at which your sound is happening does not correspond to a chain having one noise making section in it... these chains are very long and I am sure they are made so that the same spots on the chain to not match up to the same sprockets on any item in the engine...
Can you put a stethescope on that tensioner ? If the sound is coming from inside it... would be pretty easy to replace it.