Car hums at 2rpm?

My car is a 2002 Nissan Quest GXE with 159K miles. Today i hit half of a tire that was in the road. The tire went under the bottom middle of my car and i heard it bang the bottom of my car. So now whenever im am driving at 2rpm it makes that same hum noise you get when you are outta power steering fluid and you turn the wheel side to side. I have no turning issues and my car accelerates the same. Its just that darn noise that bothers me. Only thing I saw that seemed off was under the car heres this shell looking thing about the size of a big mans hand its loose. It looks like its just a cover tho. It doesnt have like a full 360 full egg shell cover look to it tho. what could be causing the noise. I called a shop and a guy said if rpms are involved could be a belt. Oh yeah the noise comes from the under the car passenger side. N i have no leaks