After installing the table legs, I have been trying to measure the beat. However, there is an unknown scattered light noise up to 400 Hz. I'm still trying to fix that.

  Here are some bullets about what happened, I'll add the details later.

• Extra noise that looks like scattered light goes up to 400Hz ( was around 100 Hz before, not from floating the table)
• One of the air spring supporting the vac tank has a leak. But it is unlikely to be the source of the extra noise mentioned above.
• The finesse of the cavities may be less than the designed value (10 000) because of the not so clean isoprop I used on the mirrors.

Note: check if the beams in the tank is blocked by wires or not.