Unusual HF resonances/noise were observed on the transfer functions recently measured on HAM-ISI Unit #4.

Multiple configurations were tested:

1. Initial Configuration: Top Mass Sitting on 3x thin Washers, one of them was mistakenly set on an optical table track (we found the washer bent) - Balancing Masses (5kg and 10kg) stacked on thin washers too.
2. Top Mass Sitting on 3 Thicker washers instead of the thin ones. Same configuration (thicker washers) for the 5kg and 10kg balancing Masses.                        -> The unusual 126Hz and 160Hz resonances are pushed towards ~195Hz, where we saw them on the previous Units. 
3. A bolt, and its washer, that were sitting in a vertical hole of corner #3 GS13's door were removed                                                                                                -> No Noticeable Effect
4. A cable, which was possibly rubbing, was set in another position                                                                                                                                                    -> No Noticable Effect
5. The washers set under the balancing masses were removed                                                                                                                                                         -> This considerably reduced the unwanted HF resonances/noise

A presentation for the results of each step is attached.

Transfer function measurements are running overnight on the ISI in its latest configuraton.