Vintage BMW Motorcycle Owners

1981 R100 67000 miles...
I've just bought my first R100.
It's so smooth. But has this annoying rattle at 3000 revs to about 3500 revs. Sounds metallic as opposed to pinging.
Seems to be left side more.
Initially thought it was from a vibration may be centre stand against exhaust but I've eliminated that.
It kicks in only while adding throttle. It dies back as soon as throttle is released back.
Using highest grade fuel plus additive.
As soon as you get above 3500 revs it's gone.
Any ideas?

Welcome! One of the first things you should do is confirm that the valves are adjusted properly and to spec. You may have some up/down movement on the rocker arms, but at some point the adjustments were limited...might have to use shims of some kind to reduce the movement. But first, check the clearances.

For the Airhead bike, that is done for each cylinder at top dead center on the compression stroke. If you do that for say the left side, then the engine is rotated 360 degrees to check/adjust clearances on the right side. Let us know if that seems clear and what you find.

There's another thing that might crop up as time goes by. The '81-84 bikes had an issue with the wrong metallurgy on the valve seats. Heat didn't transfer out of the valve faces into the valve seats very well, so the heat was retained in the valve face. Over many miles, the valve face began to deform resulting in changes to the valve clearances...typically they would tighten up. So, adjusting the valve clearances is the first thing, then you monitor them every few thousand miles to see how stable the clearances are.

Check your valves for sure, but I'd follow your original instinct and look for something rattling. Side stand, spring positions/connections, (rear) pegs up/down, bag mounts, mufflers/mounts, wiggle everything and see if anything isn't fairly solid. Loose items in tool tray; check the centerstand again. Does it do it on the centerstand when you rev it up or only when riding? Keep it at the rattle rpm and listen, does it sound like you're hitting a harmonic and something is vibrating? I'd think it would sound different than a head/engine/trans problem. It's new to you--ironing out the wrinkles one at a time can be part of the education process on keeping these bikes on the road.

Yes it's so frustrating and with the bikes so open you hear every little noise.
This appears just at 3000 revs to about 3500 revs then dissapears and the bike goes back in to smooth riding.mode through 70 and 80 mph as if nothing amis.
Thats the reason I'm looking for a harmonic vibration somewhere as opposed to an engine issue.
When standing, there are no obvious rattles at 3000 revs. It occurs in any gear when 3000 is reached, but if you throttle back it stops, it seems to be just under acceleration loading. It's hard to.pinpoint, seems greater slightly on the left side of the bike more front, but when you are riding with a full face things are not always what they seem.
Anyhow it's a great bike I love it.

You must be in Australia!