The 2991 and new fan are loud as hell but it moves some air that's for sure.

 

The Standard Duty clutch 2634 works for my D1 and not too loud. Hayden makes several models that fit Discovery, with various speed ratios. The Severe Duty 2991 is the loudest because it turns at the highest percentage of motor (water pump) speed. It is great for slow climbs up mountain trails in summer and high altitudes in general as well as idling in line at Burger King on hot days. I learned something new today about keeping clutches upright. Thanks to all the posters!

 

So upon further research, Extinct and others a fundamentally correct about storing a clutch in the correct orientation. You may get away with it one time, or even ten times, but it's like playing with fire, and you'll not know how it will spread. And furthermore, it seems that if you manage to store a clutch incorrectly and said clutch is on its way out, it's pretty much a done deal.

I also read that there is a heavy duty Hayden clutch that isn't as intense as the Severe Duty. I don't do a lot of offroading and I've never rock climbed. Maybe I should consider going with a Heavy Duty or a Severe. But then I would need to know if it will bolt up to my OEM fan or not.