It might not be so terribly different, just not so so much whine to go with the "growl" . I heard a 767 taking off yesterday, but I though it might be a C-5 before I looked. Of course the 767 only has two of the CF6 engines, Fat Albert has 4.
I hope they are not so very different. I used to work in San Antonio right under the approach to Kelly field (at Lackland AFB), which has an AF Reserve C-5 "school house" unit. I still work for the company, but now on Ft. Hood, where the AAF runway is only about 1/2 mile from the building. That same reserve unit's birds can often been seen in the pattern at Gray AAF.
Back in my Navy days, whenst I was with my boat an Mare Island in Nor Cal, I used to go up to Travis AFB just to watch these behemoths fly in and out. It boggles the mind that something so huge can stay in the air....especially when from a distance, it looks like they are going so damn slow.