At first I thought "loose speaker wire no doubt about it". Then the sound clip restarted from take off so I thought a corrupt sound file. Then it seemed to pick up in the middle of the chuff when you picked up speed.
Check your loco-tender tether though if this engine has one. Hopefully that will explain your intermittent sound. I use a dab of super glue to hold my tether in on my B6. I mean we're talking a super tiny amount. You can't even see it without tilting the engine toward the light upside down. I had intermittent sounds much like yours until I did that. I just didn't have the restarting cycle.
