From time to time I decode the weather fax images and the RTTY weather text
that come from the Boston meteo broadcast station on 9.110 Now, I'm hearing a
signal that sounds more like Sitor or PSK. I can't figure out what it is. Does
anyone have any idea of what it is ???

It may be a fax of some of the textual items they send, like broadcast
schedules, etc. It'll have a different sound that the usual charts and stuff.
Have you tried decoding it?

Bill, K5BY