What did you do to narrow it down to the EFI?
When you say "haven't noticed at idle" do you mean the sound increases with rpm's?
Anyway, when's the last oil change? Oil weight? Oil at capacity?
Maybe try some gas/EFI treatment in the gas tank to clear any varnishes, gum, etc. Then change oil...
Anything else is probably a engine mechanical problem which is beyond my help.
Need some help here guys...
