Absolutely not a leaf or anything stuck in there. It is speed sensitive, temp sensitive, and super annoying. I have just over 3k miles too.
Mine was intermittent as well. The advisor came out and listened while parked and we couldn't reproduce. As I drove away, it happened again, so I drove right back and they immediately got a tech to go ride with me. They heard it, identified it, gave me a loaner, talked with Alfa who said they have had a few reports of this (I think they must have been able to isolate it because the day they returned the car to me the noise had stopped...maybe temporarily lubricated?).
Blow motor was ordered on Alfa's recommendation. Then Alfa assigned a rep to my "case" who continued to follow up with me via email and phone on the delivery of the part.
I will follow up when replaced!