A P&ID might be useful.

This is potentially LPG flashing off in reverse flow due to a passing valve or non return valve.

When not in operation check pipe temperatures across valves which are supposed to prevent reverse flow to see if there is flashing.

Very difficult to diagnose without details and being there...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Hello,

Thank you very much for your comments. I think , LPG flashing off in reverse flow could be a possibility that we will check.
I have been trying to upload the image of the PID for two days but have not succeeded.

we also asked vendor and it is obvious, there is no issue with the compressors already checked by third party. I am not sure if somebody has experience this type of issues in other plant where the vibration is coming from external sources while machine is off.

If the external source is the reason for such vibration, then the same should influence while the compressor is in operation. But we have not seen any such case.

Any advice would be greatly appreciated.

OP,
I echo LittleInche's initial suspicion of reverse flow. Since this is a transient condition, something, ie, some variable, is changing. Any local temperature changes, load changes, pressure changes that correlate to the timing of the vibrations? My other thought is pipe design. Was pipe stress thoroughly checked prior to the compressors commissioning including pipe supports and anchor blocks? Is the compressor skid and any other equipment firmly and adequately supported and anchored?

Edit: Removed paragraph. I missed OP mentioning it was a screw type.

Guesses are all we can make for now, since you've hardly said much about what is around this compressor.
Maybe there is a recip compressor operating in parallel in run/standby mode with this screw compressor ?