Light rumble, clanking sound coming from beneath engine while accelerating, then goes away.

[Ohio JEA]

2012 Limited, 137K, just had oil changed, upon accelerating heard rumbling / gravel type sound emitting from front left side of motor. I thought it might be something left behind by the mechanic because it rumbled and clanked for about 3 seconds, then stopped. Happens intermittently at about 20-30 MPH. I have not heard it at idle or at parking lot speed. Any ideas?