I just completed an interesting experiment related to this.

When I received my wheels from Farsports earlier this year, the cassette could barely turn on the Novatec hubs. When I took the freehub apart, I found lots of metal filings in the freehub body (nice quality control, Novatec!) that were binding the freehub. The body was packed with what looked like thin green Dura-Ace grease. I removed all of the grease and relubed the cassette with mineral oil as that's what I had and that's what I'm used to using on freehubs. The hub has been running fine for about 1,500 miles.

After a rainstorm washed all of the oil out of the freehub, I decided I probably should repack it with grease as that's what it came with and it's less likely to wash out of the hub. I ordered some Shimano freehub grease online and repacked the hub. Suffice it to say, the difference in freehub noise after applying the grease was astounding to me. With oil, the hub sounded like a pissed rattlesnake. With grease it's almost completely silent. I knew there would be a difference but this was night and day. Long story short: freehub noise is 100% driven by what kind of lubrication you have in there.

Sidenote: I've learned that if you're using grease in a freehub, you have to use freehub-specific stuff. Regular grease is too sticky and the pawls can't operate fast enough. I learned that years ago when I lubed a freehub with regular Park grease and had some problems. Freehub grease is transluscent and very thin. I went to three different LBSs and none of them had freehub grease. In fact, two mechanics didn't even know what I was talking about. The stuff is easy to find online but I'm guessing this is not a very common service...