rattle noise when I accelerate

I have a 2003 Ford Expedition that started making a rattle noise last week. It makes no noise at all when I start it, and no noise when idling. As sson as I accelerate on the gas it rattles and it rattles faster if I press harder on the gas. Once I reach speed above 30 - 40 mph the rattle stops. It won't do it at the higher speed unless I coast for a bit, then accelerate again on the gas, or if I slow to make a turn, once I gas it to make the turn the noise returns. Sometimes though, it won't do it at all for a long time. I can pull out of the driveway and it will be quiet for a good 10 minutes of driving, then out of nowhere, the noise starts up again. Sometimes it's as noisy as can be starting off, and once i've been driving around a little while it just stops. This morning I put it in park and opened the hood. When I would give it a little gas it would rattle in a perfectly rhythmic speed similar to a ticking sound put more of a metal sound than a tick. The firmer I give it gas, the faster the sound goes, however, if I rev the engine with a good surge of gas, it sounds like a bunch of alluminum cans clattering around loudly with no rhythmic pattern at all. (Please forgive my musical reference- I am a musician so I guess I relate things that way) I don't know if this has ANYTHING to do with this, but I'll mention also that occasionally, but more consistantly lately I get a message to check the fuel cap. When I check it it is on there tight, but it must have some internal problem with it. I got a "check engine soon" light on a month or two ago but I never saw it again. I also have an oil leak. I don't know ANYTHING about cars, but I thought I'd throw those things out for trouble-shooting purposes- just in case any of those items are related.