I have a problem - any help ?

- the master helmet is parret to Zumo 550 - and via Zumo 550 my phone Nokia 5500 - it works perfect

- the master and the slave helmet is parret - and the connection is okey - we can talk nicely

But - when we get on the bike, and start driving - we have the enigen noise in the louadspeker in boath helmet.
i have set the helmet up - white the side tone off.

what shall i do ?