Loud Noise coming from Engine ?

I have owned my 1989 911 Carrera for 8 years . I have had a mystery sound coming from the engine, now about 5 times in that period of time .

It sounds like it is coming from the Fan Housing , like fan blade is hitting something . But it goes away when warmed up.

Like I said it has only happened a limited amount of time . The First time was about a month after I made the purchase ( gulp )  

My mechanic could find nothing . About 2 months after that it came back , but again went away .

Now I have heard it again about a month ago , and now again yesterday . I let it run for a couple of minutes and again it went away .

I have seen a couple of Posts about fan noise , but I think they were all 993’s

Very confused by the long stretch of time between the first instance , to the last . ( about 7 years ! )