My biike (a gs 500 E 1994) dtarted making an odd noise today.

It's ok when running, but is being a bit temperamental when changing into 1st from neutral.
When I stopped the bike and put it into neutral to park up  I noticed a slight rumble when moving the bike.
I put the ike on center stand and spun the back wheel with the bike in neutral.
The slight rumbling seems to be coming from just behind the gear selector lever.

I have checked the oil level : it's fine.
There is also a small amount of dark brown opaque oil on the selector lever.
(the oil in the engine is dark golder brown but still translucent)

Any Ideas?

Sounds like the chain might be a little loose.

When the engine is pulling the chain (and rear wheel) around, the tension is along the top run.  If you use engine braking, or if you put the bike in neutral and push/coast it, the rear wheel is now doing the pulling.  The tension will be along the bottom chain run, and if the chain is loose the top run will sag.

If the chain is loose enough, it will drag on the swingarm up near the front sprocket.

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The "dark brown opaque oil on the selector lever" is probably excess chain lube that has pooled up behind the clutch cover.  If you're too generous with the chain lube, the "goop" level eventually rises until it starts oozing out of the selector lever shaft hole.

See the Oil on my foot message thread for pictures and instructions on how to check this out.

What Kerry said.

Could also indicate a worn out chain or counter sprocket.  Or you could have a piece of something odd wedged in behind the cover. I once took off 'Lita's GS's sprocket cover to find a 1" stone and a rubber crawfish (fish bait I think) stuffed up in there...