Originally Posted by
Dave Therhino
It's combustor growl, or instability of the flame in the combustor producing pressure waves in the acoustic range.
What Dave said. Flame instability produces a low rumbling sound.
Most high bypass engines have some level of combustor growl during start - although the -115B is quite pronounced and the CFM on the 737NG is rather pronounced.
There have been cases where the acoustic rumble was so strong that it could case damage - forcing a redesign to reduce the instability and resultant rumble.
Back when we were doing initial engine runs on the GE90-94B (~1995), engine start was really interesting to listen to. The GE90 has a 'dual annular combustor' (DAC) and during start they'd switch between the two rings of combustors - the resultant frequency change actually sounded like a car shifting gears...