My recently purchased K100 which has a very obvious low pitched rumble at start up but it goes away as the engine warms up after about a minute.
  My first thought when I heard it was big end bearings but they would  not lessen as the oil thinned?    I put a large screw driver to my ear placing the driver end on different parts of the engine and it sounds like it is coming from the drive end of the motor or trans/clutch area?
Any ideas greatly appreciated!

1st time Beemer owner.

hi mate

if it is a pre 88 model 8 valve its likely to be the output shaft to crankshaft fixing method .these old machines used rivets to bond the hardened drive gears of the crank  to the output shaft through a rubber buffered aluminium cage to the output shaft and then to the clutch assembly cage ( think vw for a sec )

these develop some movement after many years

I believe the sharper impact ...rate of the cylinders - pistons acceleration change at cold temperatures amplifies this effect ...once the heat is up the effect is less pronounced ...and the metal has swollen a little to take up the cold temp differences on that mechanism

some have removed the output shaft and redone the fixings ...some have found a 16 valve or k1100 output shaft (which uses 12 rivets instead of the 6 usually used ) and by the way possibly nearly 10 years younger )) to repair the issue

not really an issue until the whole lot of the rivets "lets go"

if you use the search utility you will find many results  "output shaft noise "  

my own engine rattles like crazy ....but change is a coming

good luck !

Thanks for the prompt reply Charlie!   You liddle beaudy mate!    Sounds like the problem alright and now can I plan to do my clutch spline lube with some more replacement parts while she's spread open, Ooops I mean split apart. bugger .... you know what I mean!    

If you have no immediate desire to rip the motor apart, you can safely leave the rivets. Even if they fail altogether the debris doesn't get into the bores or anywhere that it can cause harm. You may be stranded with no drive but many bikes just go on tinkling away happily for the rest of their lives.

It is likely the output shaft, but not necessarily the rivets. Rivets do go south but it's more likely the tension spring b. the output shaft gears.

There is a circular spring between the two gears that provides tension. It's held in place via a post on the gear and a small hole in the spring.
Through the years/miles the hole on the spring can become out-of-round, causing slop, slap. That's probably the rattle you're hearing. 

It's a cheap part and it's an easy fix - once you've got to the damned thing. I had to remove the engine, which meant removing the trans/driveline and putting the engine on a bench upside down.

Helluva lot of work but it worked a charm and I no longer have to deal with the wincing every time I take off from start.

The rivets could well be OK and the noise could be from the anti-backlash mechanism on the output gear, which, again, doesn't usually cause a problem. You can see some of the rivets if you remove the RHS engine cover.

 My rivets look OK, but I still get the dreaded cold rattle. It's been doing it for donkeys' years so I just ignore it!

In the past it would have been known as 'character'.

Paul.


(Kyle10 has just posted a similar thought, but I'll leave my post)

I'll be dipped. Here it is:

http://hrsbstaff.ednet.ns.ca/bvogel/K100/K100.htm

Scroll down about halfway

Thanks guys.   I hate irritating whining, growling noises ... that's why I'm single.    
Now I can go in knowing where to start looking as I have to remove the transmission anyway, but really hoping I don't have to split the engine.

   What a great forum, it's like having your own BMW mechanic in your office.