Thanks for your replies! Below my replies.
Ha! This seems a cheap but very effective solution. This is certainly worth considering, if it works (and if I can get all the right materials for it), it'll save me a lot of money.
The solo II will help with the suspension but it wont do miracles if the drive is noise it will still be noisy. Get a quieter drive or isolate it on a 5.25 box.
I don't think the drives themselves are noisy. When I lay them on a table in the open they're very quiet. Somehow the R4 succeeds at making some quiet drives noisy. It's a low-frequency thumping and humming noise that annoys me, disappointing that the R4 features such a poorly thought-through HDD tray system (as HDDs are the main source of noise in my system, even if with 4 fans enabled).
A single 4TB ST4000DM000 drive would be a great replacement if that is an option. Much quieter air borne sound(a world of a difference to the current three 7200 RPM drives) and its low vibration is dampened by the Define R4's regular buffers.
A good solution if it weren't for the fact that one such drive won't match the RAID 0 performance of my 3 drives.