well here's another question

when I swapped my f20a4 for a jdm h22a (both obd2) I also replaced all belts including the timing belt and changed auto to manual tensioner
But at 4k rpm in 5 (around 120 km/h / 75m/h) there a strange humming comming from my engine bay, sounds like someone is hiding under the hood and playing with a crystal wineglass or something and stops when I go above or below 4k rpm
figured it might be the timing belt that's off, but everything seems to be right.

any ideas?