When I started our car it made a bubbly knocking sound then ran fine :S

Hey everyone, 

Please excuse my ignorance & silly sounding discription.

The other day when I started our car it made a strange bubbly, knocking sound. When I drove off it ran fine.  

Tonight when I tried to start it, it made a similar sound and did not start. I tried again and it did not turn over. I tried again and it started. I drove home and it ran nice.

Any ideas on what should be checked? I do not see any warning check lights.

Thank you for the help & time :) 

~Meredith