2005 FE450 engine noise - left side?

In my experience those engines usually make some noise but it's hard to say without hearing yours specifically.
As for the oil leak I'd check the breather hose they tend to crack if you over tighten the clamps.

Rod knock? Rod bearings may be going or perhaps the mains. In any event, I'd open her up to look around before tempting failure. Best to get out in front of it and avoid having to replace parts that would have otherwise been unharmed, or worse, suffer catastrophic damage that may hole your case or irreparably damage the head.

the noise I most often hear is that of the little needle bearings that hold the starter gears. they can be felt through the cover at about 12 o clock middle. they can also 'pulse' in their noise. wa-wa-wa-wa at tickover.

Taffy

Yeah, as stated above, those engine are quite noisy to begin with, and it's not always easy to hear the difference between normal noise and something thats not normal, unless you know those engines very well. Normal noise usually either from the clutch, starting meachanism or the waterpump/counterbalancer gear, or all of them at once. Bad noises can be main bearing, bigend, timing chain, too much valve clearance or worn piston to name but a few.

Its easy enough to check the timing chain and valve clearance, draining the oil and checking for metal in the oil can indicate main bearing or conrod.

How many hours running time on the bike?

Oil leak from the rocker cover is as stated above either from the breather, the valve inspection covers, or the rocker shaft plugs. Could also be the main rocker cover itself.

Oil level should be at the lower part of the sight glass.