Loud noise in engine on 2004 Mazda B2300

When first excellerating engine feels like it is missing (not very noticable though). When speed is about 40mph the engine produces a very loud "roar" sound- no knocking or pings, just roar. The roaring sound will stop when speed is reduced but begin again as speed in increased. There is no loss of power, no exhaust smoke or smell. Regular maintenance is kept up and there is only about 60K miles on truck. I am the origional owner and do not drive this truck very hard. Any ideas?

Asked by for the 2004 Mazda B2300
2 answers
Have a shop check your fan clutch because when these go bad they will make a roaring sound as engine RPM's pick up, about $58 at www.napaonline.com
Cut your catalyser, they tend to clog big time