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ZFs have always been noisy especially the early trans which has the reverse lock out collar, so the noise maybe normal or the dual mass flywheel may be going out.
As for Bill not knowing about the effects of performance enhancements, I find that hard to believe, considering he builds super strong boxes for racing... perhaps he doesn't speak of it, because I get the feeling he works from empirical data; if he doesn't have it, he keeps his mouth shut.
Bill and Kurt have both built plenty of upgraded boxes for racing with external coolers and all the usual stuff. I would be comfortable sending my trans to either shop.
Kurt personally owns 2 ZR1s, one is a radical 415 with a direct port nitrous setup the other was a 408 with a direct Nitrous setup but going to a 368 TT.
I can also speak from first hand experience do not trust a local shop to rebuild your ZF even if they are recommended by a race team.
I'd hit the search function here and look for the thread that had ZFs for sale at $1800. Something about a Chevy parts warehouse wanting to get rid of these trannies taking up space in their supply house.
I bought one of those last year. I paid 100 over the final auction price to the dealer (his profit was 100). I told him I would be willing to bid 2000 for one plus give him 100 for his time. Shipping was included if I remember correctly.. GM only sold ~200 out of the thousands they had in stock. you might be able to find a dealer to sell you one for 2k. Keep in mind these transmissions are NOT New. All are rebuilds. crate has a rebuild sticker on it.