Tacx Flow Smart + Zwift = horrible noise (not the rider)

Happy New Year to all.

I have recently upgraded to a Tacx Flow Smart for my garage Zwift sessions.

Today was my second ride and yet again a horrible noise. Set up is a Specialised Allez on the Tacx Flow Smart with Zwift on an old Ipad Mini 2. I ensured that the TT was calibrated before both rides and the Tacx App said it was.

The TT is paired up ok however periodically and usually when the ride goes to a negative gradient something is making a noise. Sounds almost like the noise I hear on more expensive bikes coasting, the noise usually stops when the gradient gets back to 0 or above. This doesn’t appear to happen when outside of Zwift. I would say the noise is like the noise I hear from expensive bikes free coasting but sharper and louder.

What’s going on here. Have I got a dodgy TT, an issue between ZWIFT and the TT or user error. I am maybe 25km in using it.