Weird ticking on C200 1996

Hi again, This time I want to ask you all to help me resolve such problem - when I start engine and turn heating on, after a few seconds some weird clicking
comes form right area of engine (front of passenger seat). It just ticks, ticks and ticks - and stops a few seconds after I turn heating off. Now, is it a broken relay? Is it something to really worry about or should I just let it be? Thanks in advance, Andrzej
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