Thanks to every one for your input. We'll now drain our tanks, put 97 octane gas in one of the bikes than put a octane booster and a injector's cleaner.
Will report back witht the results. Keep the advice coming in the meantime.
One thing that I forgot to mention is the oil leaking/running down most likely coming from the air-filter chamber's draining hose (usually used for letting fumes scape, i'm guessing?). This is happening to both bikes.
The 'metal' pounding noise coming from the bottom part of the engines, where the letters 'BMW' are (bellow the twin cylinders) on the left side (chain and sprockets side).