Ted,
Thanks for the detailed instructions. I can definitely follow those if I need to at a later date. For now I'll leave the adjustment to the PB, but at least I'm empowered if it starts again.

My problem is so intermittent, I share this in case it generates any other thoughts. The noise only starts after the heater has been running for a while (30 mins plus) and seems to occur some days and not others. I'm guessing the intermittent sound could be because of atmospheric conditions, temp, pressure and RH. For example on Saturday it was howling after 1/2 hour of running and it would fade in and out, but Saturday it was mostly noisy. It's a shrill almost painful harmonic. As I removed the cowling on the side of the blower inlet the noise stopped and as soon as I'd put the cowling back on the noise would come back. It's hard to believe that the cover could change the airflow by that much? However from what you've told me about how sensitive the adjustment is, it would seem to me that I must be very close to the ideal setting.

It's a beautiful fact that I'm worried about my heater here in Rochester NY on October 1st and we're still swimming outside. It's been 75-80 almost every day and expected to be so for the next 7 days. It's been a great first year for our pool, swimming since May 9th and counting.

Thanks again.
Andy