David,
I never had any problems with Kato Customer Service. I even had a chance to meet them personally at a local train show. Nice crew!
I've also heard other prople praising them. Granted, in one instance it took several tries, but at the end that person ended up with a brand new loco.
Are you accurately describing the issue to them? It sounds like a miscommunication of some sort. I realize that one of your locos works ok, but are you maybe using some sort of an oddball throttle? Is the noise always present? Or does it only show up at certain speeds? Is it present when loco is running by itself?
I have 3 of those locos and I ran 2 of them for several hours on an N-Trak layout pulling a long pass. train. They are typical, very high quality and smooth running locos I come to expect from Kato.
While back, both Model Power (first) and Life-Like (second) produced F40PH. Compared to Kato's, both of them are like simple toys. Those locos used the old and tired mechanical design from the 70s. Life-Like had a better motor, but otherwse, it was a poor mechanism. Both in appearance and mechanically, Kato unit is like a Cadillac vs. a Yugo.
When Model Power ones were the only ones available, I used their body shell on a Bachmann GP50 mechanism (which was fine tuned, as much as it can be fine tuned). They actually ran quite well. Of course, I also somewhat detailed them and repainted their bodies. I had lots of positive comments about them. I still have them.
Oh, one of the brass companies (Hallmark?) made F40PH in the late 80s or early 90s. It looked good and ran ok (just ok). Typical brass loco of that time.
Again, Kato version is leaps and bounds better than anything else on the market.
While I haven't totally dissassembled the loco, loking at the parts diagram, it uses a mechanism slightly different that other Kato locos. Again, since only one of your pair has a problem, that fact shoudln't matter.
I would say, try emailing Kato to see what they can do for you. Or better yet, give them a call and talk to the service department. They are very helpful and I don't see why they woudln't work with you.
Peteski