All the above, and possibly a failing oil pump. Developed that same noise, let it warm up every time and then one day just putting around I heard a faint knock, 2-3 miles later it was unmistakable. 5 miles later (had to get to work), it was pretty obvious the motor was done.
Basically, start saving money for what could be an engine job very soon.
1991 HF - previously '90 B18A1 now '97 B18C1
1989 Si - running parts car/gone