A170 headset gives grumble during calls

After replacing a plantronics 540 with the snom A170 (base is a D715 with expansion) the customer complains about grumble on the headset.
We had it replaced for a new one, but the grumble stays. We have tried several spots to place te base, but it stays the same.
Does someone in the community recognizes this problem and has a possible solution?