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For sound insulation everyone recommends mass loaded vinyl (MLV), but that barely made any difference in my case. MDF/wood is thick and heavy enough on its own.
Foam easily eats high-pitched sound, but low-freqency harmonics almost drove me mad. 5700RPM HDD turned out to have 95Hz harmonic that spread through everything, including foam, felt, rubber feet and my floorboards!
High freq needed closed foam, and the low freq needed layers of butyl rubber mats (for car sound deadening).