1998 DD 4WD 5.2 Knocking Sound from Wheels

[jrandallc]

On a road trip this weekend, I starting noticing a knocking sound coming from what sounds like the front driver-side wheel. It is more pronounced at lower speeds, but seems to go away at higher speeds. Any idea on what this would be? Also, while I was checking the engine oil I noticed that I left my oil cap off the last time I added oil. Are there any ramifications to running with no oil cap for a couple of weeks? I checked the oil level and it was still fine.