I seem to have a slight metallic farty sound

when the engine is underload, also at high revs there is a slight whistle.
I've only just replaced the driver's side manifold gasket (V6) to cure a leak when cold - which it has. The gasket was in a fairly poor way but only leaking a little.
I've also found a hole in the back box and replaced that too.
But there's still a leak - I think.
Stationary I can't hear it at all. I've spent quite a bit of time listening whilst someone puts their hand over the end of the exhaust pipe - but nothing. Rev'ed away and nothing.
The EGR looks and sounds fine.
I think the noise is coming from the passenger side and I can clearing hear it with the windows open when accelerating in built up areas (sound reflecting off the houses).
My questions are:
1. Do the manifold gaskets tend to fail at around the same time?
2. Is the sound typical of a manifold gasket or is there something else worth checking?
3. If (as on the driver's side) all the exhaust studs come out when I try to undo the nuts is it possible to change the manifold gasket on the passenger side without moving the engine or removing the head?
Cheers