I have two whistle tenders, 2671W and 6466W that I am cleaning up. The 6466W works great, but the 2671W whistles okay until I power up to maybe 75 - 80% throttle. From there to full throttle it stops whistling because the whistle relay drops out. If I release the whistle control, the relay will pull in but will immediately drop out. No problems at lower throttle. Again, no problems with the 6466W across the entire throttle range. I am using a KW transformer with nothing else running.

Thanks for the suggestion Larry. I have a lot of other cleaning and maintenance to do on my collection, so it will be a while before I try swapping the relays, if it comes to that.