"...what is the difference?" Hard to describe in words, as is the difference between any two sounds. EMD did used to be 2-cycle and run at higher RPM, but is now 4-cycle, as is GE.
But still, they just don't/didn't sound anything alike. The ultimate inspiration for GE prime movers were Alco designs; GE's sound, to this day, is Alco-like. Chug-chug. They even smoke a bit like them. I've heard or read the comment that "inside every GE is an Alco
trying to get out".
So, at least until the recent past, when EMD went 4-cycle, the EMD and GE sounds were
unmistakably different. But you need to pay attention to the real thing.
I had an MTH EMD SD-9 upgraded by someone else to TMCC/RS years ago, and the loco was sent back with an Alco sound chip. I immediately called them and they sent a replacement chip for the swap. Just wrong.
