strange engine noise

[e30-320i]

i know its a long shot as its hard to describe a noise.and you cant hear it yourself but with my bonnet up i can hear what i can only describe as a bubbling sound coming from under the rocker cover i've heard it before and not though anything of it but a mate popped the hood the other day and the first thing he said was "what the f#=ks that noise" he's always tinkered with engines a he says he's never heard anything like it.
does anyone have any idea what it could be

[kam-325i]

Might be a loose nut or bolt being thrown about ?????