BVC
I was very nearly in a similar situation on Tuesday and i've got 6 years (every day) experience.
We have been running flat out for four months now and did some drain jetting in Derby on Saturday & Sunday night, machine sounded very gratey, a completely different tone & pitch to normal, put it down to the drain jetting nozzle being a different set up.
Went to check the engine oil on Tuesday and it was very nearly empty, filled her up and hopefully no real problems, could have been a complete engine seize up.
Even the knowing get it wrong, sorry to hear about your misfortune, don't give up on the pressure washing, if you have an ongoing contract that pays keep at it, find a way and build on it.
I am sure you are feeling crap at the moment but onward & upward eh!!!!
Rob