89 regency 3.8 l.

I have an 89 Olds. Regency 3.8 liter with 100,000 miles. That when it warms up it sounds like something is loose internally. I have checked the trans. and the exhuast out and all is good on that part. I pulled the oil cap off and it sounds like it is coming from there. Everything I could check externally seems like it is tight. Thanks for your help.