OMG I only stay in 1st until 5 mph, stay in first long enough to hear all that is waiting for the cager to kiss you tail (well in CA that is) That is the sound that your engine is screaming for mercy

It sounds like your carburetor is clogged up with varnish. Has your bike ever sat for an extended amount of time?

These bikes run quite smooth and should accelerate smoothly from idle to redline.

You are pushing in the choke?

As suggested by SavagedDude, 1st gear is to get the beast in motion; the engine is politely trying to take the transmition apart I suspect.  I think you might be hearing a metalurgy stress test.

Thanks for the input folks, okay now I'll get my story straight. After reriding the wife's Savage and focusing on the actual noise and conditions, here's the scoop. The noise sounds somewhat throaty beyond "half throttle. AT HALF THROTTLE, first gear is actually fine at a shift point of 15 mph (owner's manual states 12 mph for first) so no excess RPMs, second gear likes 25 mph, 3rd gear likes 30-35 mph, somewhat above the "shifting up schedule" but well below the max PRM limits. Checked out the air cleaner, it's fine. Ran the bike at half throttle in 3rd gear about 30 mph and cranked on full throttle, very throaty sounding without rapid acceleration. My Mr. Max (V-max) will put you into instant "Beam me up Scotty" warp speed  under the same conditions. Am I taxing the 30HP Savage when asking for full acceleration in the lower gears? Savage had 1500 miles on it when bought in fall of 2005, has 2600 miles now. It's a 2000 so it may have sat for a while. It runs absolutely fine if you pretend it a scooter. I had various bikes, all twins or V-4's, some below 650cc, all accelerated aggressively. I'm not sold yet on this single cylinder concept. Any additional input would be helpful since the wife really like this bike. Don't get me wrong, I not against the bike, it just seems like it should perform much better when accelerating.

the wife's bike accelerates just fine! she does a pretty good job of keeping up with my HD, in most circumstances. i have accelerated her bike hard enough to slip the clutch pack but we only run premium fuel and i have completed our carb/exhaust mods. i cant see that dumb speedo when i'm riding, so i shift with "the seat of the pants method", with no detectable rattling.