I had exactly the same problem. Drove me crazy, because I only used my Strida few times and it used to be silent. At first I suspected the bearings of the bottom tube, which are also not okay, but this could not explain the sound.
I then checked every bolt I could find and found the source of the problem. The steel ring fixing the rear wheel had worked itself into the aluminium frame. This ring appears to have sharp edges and since aluminium frame is much softer than steel frame, it has no chance.
After removing the ring I did the following:
- Carefully remove the burrs from the alu frame (for the largest part),
- Used fine grade sand paper and a water stone to remove the sharp edge of the ring,
- Then I mounted the ring with some copper grease.
Since then it works like a charm. No sound whatsoever and I am happy again.
Hope this helps you out as well.