I've actually had the same problem with my Morgan 4L, but not my 3C. One thing I noticed is that the 4L is noticeably longer than my 3C (maybe 1/4" longer), with the 4L having a serial number of 15055 and the 3C having a serial number of 25233. I'm not sure if this is a function of the different models, different ages, or both model and age. Oddly enough, I noticed a similar burble using a Penzel Mueller, which I hadn't expected considering that it's shorter and seems to have a larger chamber than either Morgan. My (highly uneducated) guess is that it's dependent upon the ratio of length to chamber volume for the burble, while intonation is dependent upon each individually depending upon what region of the octave is being discussed (for example, mouthpiece length relating to palm keys intonation, and chamber size relating to bell keys intonation).