Yes. I have only seen two reasons for a whistling sound on inhalation in a balaced piston regulator and both are related to sympathetic resonance.
The first and most common is inadequate lubrication of the HP O-ring. It causes some vibration in the piston as it moves and this vibration then resonates in the LP hose making a fairly loud sound that does not always sound like it is coming from the first stage. Mk 20 and 25's can have a similar sound caused by the upper bushing being installed upside down which again causes some resonance.
Many divers and more than a few techs claim reversing the spring will resolve a whistle in a balanced piston reg, but what is really happening is that the o-ring is getting more lubrication delivered to it when the piston is removed to flip the spring and the lubrication solves the problem not the change in spring orientation.
The second cause is some harmonics between the first stage and a J-valve. This is obviously a lot less common than it used to be. I had one Mk 10 owner that basically went nuts over the problem for months. The reg just would not whistle in the shop on the bench regardless of supply pressure, but the customer continued to have intermittent problems when he dove. Since he was a spearfisherman, he was not pleased with the noisy reg. After much trial and error, several visits and noting the same empty tank brought in on a couple of consecutive compliant visits/air fills, it finally occurred to both of us that it only whistled when attached to that one particular tank. The seat/spring arrangement in that particular J-valve resonated when attached to his Mk 10 after the pressure fell below about 1000 psi. A new valve solved the problem.
