Only seems noticeable when the gearbox changes from first to second gear but sounds like a sink being emptied when you pull the plug and you hear it splashing the grid outside... All within a second or so... Someone suggested a dump valve of some sort, well? If it is a “dump valve” is it normal? And what's it actually doing!
This has been raised once before with this contribution by me but no one answered so thought I’d raise it once again. Thanks in advance for any suggestions. 57 HSE AUTO with H.I.D's - Grey with Black leather. (H.S.E. Heightadjustable Seats Extra )
My Aviation website - UKHelicopterPilot.com

try searching +strange +noise

been covered many times before