Thunderball!!
I, too have seen what you have seen. I was sitting in window seat right neat the no.1 eng of a UAL DC10. It appeared over the engine, and I would best describe it as having the appearance of a very hot jet of air blowing in a straight line (as we were moving about M0.8x, obvioulsy any air escaping would've had a curved appearance). The "jet" would have only been around 5 mm in diameter. Truly amazing stuff to see! I have not seen it since.
ODL
P.S. Very different to the enveloping cloud of mist seen in many photos, as it was the very first are of supersonic flow {Mcrit}. Therefore I can only assume that Mcrit on a DC10 is below crusiing speed.