Check engine light stays on for a while then goes out.

Hello; it appears that this along with your OnStar concern represent multiple failures or problems with your recently purchased vehicle. If you purchased it from a dealership of some kind, I strongly recommend you visit them with your concerns and seek their assistance. If you bought it from a private party -- then unfortunately you are on your own with diagnostic and repair costs. Simply put, you need to troubleshoot each concern and then resolve it with the appropriate repair. What testing has been done for your check engine light? Physical inspections for your fluid leak and clicking noise are also needed. There really is no advice or guesswork here in this forum that could help you further. Good luck!