Hi bcikota and thanks for your help.
It's hard to describe the noise but It's similar to a waterfall (a small one that is

). No cracking sound except when running it at < 55Hz (motor noise).
You're right, I used three rotaries (one Q Fitting under the reservoir and two 90°).
I would have liked to link both of these with straight fittings but it's just not possible (not enough room without either kinking the tube or the pump moving around).
As you said it could be the pump struggling to get enough water and eventually leading to cavitation. That would be strange with a 150ml reservoir anyway.
I may remove the optical bay and move the reservoir up to free some space but there's no way to do so on the 350D, unless I cut it.
Another solution would be to use the Aquacomputer D5 (qui smaller than the Aquastream but maybe noisier).
One way or another I'll wait a few days, drain the loop, try straight fittings and eventually change the pump.
Regards.