When I ride there is s distinct change in the pitch of the engine. The change starts at around 3300 or 3400 RPM peaks at 3500 RPM and then returns to normal somewhere around 3600 RPM. Other than this the sound of the engine is normal and very smooth. The bike has approximately 100,000 Km on it (1995 K75RT). This happens no matter what gear I am in and the change is always in the same range of RPM.

I thought it was related to the valves. I spoke to the original owner and he said he noticed the same thing during his ownership so it has been going on for some time now. He suggested it may be the cam chain tensioner but not sure.

Any thoughts on what could cause this or if it is something I should be worried about?

It is possibly being caused by slop in the drive for the balancer shaft. 

My '94 K75RT has an annoying rattle that occurs at about 1500 to 1700 rpm.  It occasionally happens at the harmonic of that rpm range ie. 3000 to 3400 rpm.  For some reason the slot in the drive is about a mm larger than the dog that engages it.  Any irregularity in the balance of the cylinders that results in even minute variation in rotational speed will make the dog rattle in the slot creating a metallic clanking sound.

The slop appears to be intentional, neither part shows any signs of wear causing the gap between the parts.  My bike now has close to 97,000 miles on it, and there has been no bad effects from the noise other than making a very nice machine sound like a bucket of bolts when increasing rpm from idle.

I have tried several approaches to making it go away, but the best results come from carefully balancing the throttle bodies, adjusting the valves to have as close to equal gaps, and periodically running a bottle of fuel system cleaner through the injectors.  Still, even though it can be made marginally better, I can't make it go away.  I guess it's just part of the bike's charming mechanical personality.

Thanks for the insight .75.  Not sure we are dealing with the same issue. The sound I get is more rotational like a whirrrr that changes in pitch as it goes trough the range. I do not hear any rattle. 

In Quebec we are quickly approaching the end of the riding season and checking the valve clearances was on my todo list for the off season as part of the maintenance for next season. I am not sure what is involved in balancing the throttle bodies, will that require any specialised equipment? I will have a look at the tech page to see if there is any more detailed info on that. 

The idea of having a look at the timing chain had crossed my mind, but that is a tad more than I had planned for off season maintenance. Still, I will probably have more than 4 months to try and diagnose the problem so I might just do that.
I am lucky to have a garage even though it is not heated. Might just require the occasional dram of scotch to fend off the chills.

Automotive stethoscopes are pretty cheap and a worthwhile addition to your arsenal. A long handled screwdriver can also substitute for the stethoscope. Try and determine where the noise is coming from.
Regards Martin.

Have you checked the monkey knutz in the alternator drive clutch? They may be worn.

Possibly the alternator bearings are getting worn.

Thank you all for your comments, I will be following up on Martins suggestion re stethoscope, not sure if idling at 3500 RPM will give the same result. Don't think I want to check with a scope at 3500 RPM (is 80KM/hr on the nose in 5th gear). Checked the Monkey Knutz but only found Brass Balls in there.

I am still considering having a look at the  on the cam chain tensioner as suggested by the previous owner. 

A whir occurring within 3300 to 3600 rpm? Beware!