DS416 knocking sound

[Quantillion]

I recently had one 4TB disk crash (out of 2 x 4TBs) on my 4-bay DS416. So I decided to replace the lot with the max 4 x 8TBs. I've put one in so far to "Repair" & clone the remaining original loaded disk. All looks good, but there's now an audible & constant knocking sound that I hadn't heard before. And I can feel it when I put my hand on the box.
It's about 3 years old & I was hoping by expanding the volumes it would go at least another 3, but this sounds ominous.

 

[jeyare]

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