Hello. I'm new here, and I'm trying to help a friend find a good used S63L 4Matic in Japan.

There is a issue on a car we're looking at, that I was hoping someone can shed some light on.
The car is a 2013 S63L 4Matic LHD.

With the engine idling, and sitting in the passenger seat with my foot flat on the floor, I'm feeling some kind of vibration. It's not directly relative to engine RPM, and it feels like a double tap kind of vibration.
So it feels kind of like this(I'm sorry for the poor explanation):
nothing, nothing nothing nothing, tap, tap, nothing, nothing, nothing, nothing, tap, tap.
When the engine is held at a higher RPM, the timing between the double tapping(the nothing part) does not change, but the tapping speed seems to be faster.

The used car dealership had a shop look at the issue, and has told us that it's a non-issue, and it's just a heater pipe under the floor going to the rear seats causing the vibration.
I'm not sure if I buy the explanation, and I was hoping if someone here had a similar issue or explanation.
For a car of this class, a vibration like this can't be normal, I think.

Thank you for your help, in advance!

-Kei