The yellowish foam you see if moisture and it is very normal. Once the engine warms up the moisture works its way out of the mix. the only place it really collects is the fill cap.
To the cat who never saw yellow foamy oil on his engine oil cap............start the car in the morning, drive down the block, come back and shut the car down. Wait 5 minutes then pull the oil cap off..........
As for your other question...what type of metal sound is it??? You need to try and be a little more precise on explaining it. If you could capture some sound clips that would be best. Is it metal on metal? faster or slower with RPM?? coming from the engine or suspension?? Can you feel anything in the steering wheel? Is it metal on metal or metal on something else?? When did you start to hear it? has it gotten worse? It could literally be a million different things.
