timing belt maybe?

[mattferguson14]

yeah, not a knock, definitely a slapping sound. Almost like a lifter sticking or something. But it sounded like it was coming from the front of the engine. Now the noise is gone. It was there for just a sec when i started it up earlier to drive home from work but went away almost immediately and haven't heard it since.

[Cobrajet]

yeah, not a knock, definitely a slapping sound. Almost like a lifter sticking or something. But it sounded like it was coming from the front of the engine. Now the noise is gone. It was there for just a sec when i started it up earlier to drive home from work but went away almost immediately and haven't heard it since.

Possibly your serpentine belt tensioner or idler. Mine made some seriously bad noise for several days before I finally figured out what it was and replaced both the idler and the tensioner.