Can a control valve produce  high noise (more than 85 db) under low pressure drop, say 20 psi or 30 psi?

Your question is too general to be answered without a specific fluid given, and also a specific valve including a full description of the pipeline before and after.

 Trying to be systematic:

a) Water: probably not, roughly checking out a couple of common control valves from max given pressure to athmospheric.(eg. worst case)

Reason, as you indicate: noise is created by obtaining cavitation and /or flashing conditions. Control valves are in addition built to give best possible flow and regulation, eg. lowest cavitation possibillity.

Even using 'non control valves' (worst construction type valves for control, for instance butterfly) you could probably provoke cavitation to be heard, but not a full cavitation at highest noise level.

If you cross the vapor pressure to the low side, you will flash.  If the valve recovery coefficient is high enough to recross to a pressure higher than VP, you will revert to a liquid.  If the +/- 30 psi drop crosses below then back above the VP, you've got it. b) Other fluids, including gases. Rephrasing of question: Is there a fluid that under a given pressure and temperature, could reach flashing or cavitating conditions by a pressure drop of 30 PSI through a given control valve?

My feeling is 'unlikely' for this one, but I am not able to 'prove' it.

Others out there better to comment on this? - We are anyway moving into 'theorethical engineering'.

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