Severe vibration in x-axis

I have a Tarot 680 pro quad copter, 1550 propeller and 6s battery, KV300 motor, it suffers from severe vibration in the X-Axis (even not in Y-Axis).

The Anti-vibration Plate is like:{"sku_id"%3A"66910978050"}&pdp_npi=2%40dis!CAD!6.26!3.19!!!2.48!!%402101d4a716668615195995106e4c53!66910978050!sea&curPageLogUid=8gXiwxbLOOcD

The log can be seen in

From the acceleration power spectral density plot, there is a peak value at around 38hz and it’s constant, not correlated with motor RPM it seems. I have tested the balance of all my four propeller (run one by one and feel vibration by hand) and they are okay.

What could be the cause? How can I test it?

update, problem solved. The problem come from the anti-vibration plate, when I dissembled it and mount the FC directly with a 3M adhesive tape, everything works fine.

The newest log: