I grease the u-joints every other trip, along with the thrust bearing. I grease the zerk inside of the noozle after every outing.

Patch the "grumbling" or growling could be Ujoint/drive line noise but it typically it is prominent at idle and dissapates as soon as the impeller see's a slight load , the fact that it is getting a litttle warm and is slow on the initial "uptake" is a concern for sure and suggest some possible internal issues ,as Omar said droops are notorious for this particular problem (if equipt) and X'2 on the injestion of grime from towing etc Call Josh or myself if we can help you further diagnose this Tom

re Pump grease AT suggest a small amount added every 10 hours of operation

Thanks for all of your comments. The pump is equipped with a turbine trim without a droop. Also the noise gets louder when the pump finally grabs and starts pumping. The top speed is down from 64 mph to 56 mph on a hand held GPS.  I have gotten four good years of boating on this pump since the last rebuild. Most of the time has been split between Parker and Blythe. Approx 350 hours since the rebuild.
I am going to pull the pump after this weekend. Would a droop help my advantage 20.5 classic closed bow gain more speed? Thanks for all of your help. Patch

thats what i was curious about too Ron 

Growling? Can't be an AT must be a Panther. But seriously get it looked at by a pro then decide if you want to fix it yourself. I would take out the u-joint and check tolerances and lub while out of the boat (replace bolts while you're at it). spin the pump by hand and feel for grit. Snug without a grind possibly ok but could be a wear ring, jiggles for and aft it's the bearing, spins like a top, wear ring maybe or it might actually be ok but probably needs lube since 90 wt will make it hard to spin. If anything feels wierd pull the bowl and inspect the impellor. Like I said have a pro look at it.