One of my 7904s just crapped out with HV arcing inside the PS unit - very loud and obvious. I had just fixed it (it had other problems not HV) back in November, and I'm convinced the failure was due to high humidity damp and cold conditions out in the garage.
If your new unit is quite filthy from what you've seen so far, the inside is probably pretty bad too. If it's working now but making discharge noises, you should open it up and clean the HV section (and the rest too as needed) thoroughly, then look for evidence of arcing or component damage. Chances are it should be good to go after cleaning.
I think that mine unfortunately ended up damaging something. I should have shut it down immediately but I thought it might clear by itself. No such luck. It still showed a messed up display at first, but then went blank - bad idea to let it go.
Ed
