Her Smart is a 13 plate and HH makes no noise at all. Has your HH always made that noise?
My (ex) Smart did, yes. I guess it was some kind of RF interference from the HH device.
Glad to sell it really although the main reason was the atrocious despicable state of the roads in Sheffield and how desperately uncomfortable the suspension was on them. The body is so light, the weight thereof doesn't allow the shocks to absorb the potholes, the drop or up-push transmits into the body making it buck and the ride uncomfortable.
R-V-M