98 350 Vortec

My 3/4 ton K2500 has a 350 Vortec with 71,000 miles. I am the 2nd owner. The previous owner ran true duals with twin cat conv., a throttle body aluminum spacer, K&N air filter for sure. This truck gets decent gas mpg @ 13.5 - 14.5 but has very little low end power compared to my 94 1/2 ton 350. I run 285/75/16 tires which hurts stock gears on low end I know. Now, here is my concern, this summer my engine or tranny would roar like an airplane in 1st and then slowly fade before shifting to 2nd. This happened not all the time but quite often especially when pulling my boat. I thought/hoped auxillary fan clutch. Flushed tranny and noise only happens when taking off in first when the engine is cold. Now problem number two. Engine knocks/pings with 87 octane but smooths out with 89-93. When taking off from stop, this truck runs well but seems to have a lull/ sound change/power shortage around 1900 - 2200 rpm's. I know this is one of those things that you have to drive it to understand but I just need a starting point. This "lull" has no rpm change, in fact, while increasing speed the r's stay steady at 1900-2200 for a second while increasing gas. This seems to only happen in 1st and 2nd gears. By the way, I did notice I had a cracked exhaust manifold on the drivers side if it matters. Thought it might be confusing the computer somehow. No engine light warnings are present. Also, I love my truck but it wont't spend the tires on wet grass. Thanks in advance for all advice. Skeet51

have you look to see if your rear end has been change?? It possibly could be your tires ur running...

what size is your exhaust

airplane roar clutch fan is bad.

i know indexing your spark plugs will help some with fuel economy,hp, and knocking do to low octane, though it may not be the main problem. by indexing i mean make sure the electrodes inside the cylinder is facing the outside of the engine by marking the plug with a Ssharpie marker wher the plug wire connects so you can tell if it is at 4 oclock or whatever it should be

If it's only doing it when it's cold it's probably the fan - They're all noisy as hell when loading just after a cold start.