What is a :"black line of presssure wave"?

[gadjo]

Just fininshed reading an article about a old Qantas 707 being ferried back to Australia. At some stage during the cruise the writer sitting in the cabin could see " the black line of the pressure wave oscillating around number three engine".
Would anyone be kind enough to bring a bit of light on the matter.

Thank you.

[Spooky 2]

shock wave?

Only a guess but I think he refering to the Mach shock wave which can seen during certain atmospheric conditions. The reference to the number 3 engine is a little confusing other than this condition can usually be seen on the inboard portion of the wing.

[barit1]

Here's a previous thread. There may be others too.