I also changed the column and guard cartridge too. This didn't help either. Any suggestions would be most welcome. TIA!
Without knowing what model of DAD you have, what I can say is if it is noisy, I would try another flow cell if I have one. Make new mobile phase (if you just dump mobile phase in the bottle every time, change solvent bottle and make fresh mobile phase. I would also look at my pressure to make sure my pressure is stable or if it fluctuates a lot. If it fluctuates, look into your check valves maybe or your degaser.
Based on your latest results, I would say that nothing is fundamentally wrong with the optical unit in terms of noise, so I believe this is more an applicational problem - your passed quick noise test points to that direction.
The error 1050 with the failed Holmium oxide test is a problem though and needs to be fixed. Make sure the flow cell is seated correctly and has no contamination. If the HOX test still fails, you need to call an Agilent service.
Rechecked seating of flow cell and also swapped cells from a different detector. Still failing holmium oxide test. Looks like there is nothing else to do but call Agilent. In the interim, I may swap out the faulty 1260 DAD with an 1100... Not sure if the older model will be readily configurable.