Not just a Jensen #50 whistle modification, but that same set-up will modify the whistle on just about any other Jensen boiler as well.
I've always been sort of curious why Jensen never developed a better whistle, but then again, the standard Jensen whistle always overloads and shuts down the sound recording on my videos, so I must assume that it is actually a very loud whistle at a frequency that I can't quite hear, but my camera can?!?!
They are very high pitched and tend to leak a lot stealing power from the boiler.
Actually, you can generally get Jensen whistles to seal up, just like the steam line throttle valves, by working them back and forth repeatedly, under some moderate hand pressure, until they seat into their cone better. If that doesn't work, you can go to the next more aggressive step by doing the same thing with a little fine lapping compound in the joint.
The other thing I always do before starting up a Jensen, even when I think I've got all the valves well seated, is to put a fraction of a drop of steam oil on those cone seats by forcing the joint open against the spring pressure and placing a toothpick tips worth of steam oil into the joint.
Same with the S/V by the way!