My 1377 at the first pumping has always made a sort of cracking sound.
Next pumpings are quiet. What It means?
Most likely it is the check valve. I doubt there is anything to worry about. These things just make strange sounds when pumped or atleast some do.
Mine took quiet awhile to get broken in. When I first got it the inlet valve would hardly open at all, it was extremely difficult to pump it up from the first stroke onwards.