I have a Saab 95 2.2 TD Linnear. About a week ago I noticed a booming sound that starts at about 30 - 40mph and disappears at around 70mph. It is a fairly constant sound from the central front (ie: it doesn't get louder or change pitch the faster I go). It does seem louder when I take my foot of the accelerator. The engine seems rougher and performance has dropped off I think.

There are no warning lights coming on.

I have taken a look under the bonnet - can't find any noticeable problems except an oil leak around the clutch connection. The air inlet hoses seem intact and connected.

Any thoughts or ideas on where to start looking please?

Hard to say without hearing it. Is it possible a splash shield or wheelwell is loose and at high speed it is slapping wildly under the car or into your tires?
Hoping the engine isn't making boom noises. I had a shield bolt in front of my wheel on the passenger side go and it would pop loudly like a cvshaft when i would let off the gas. That is why i ask.

the popping was constant and would change in speed depending on my speed.
I would inspect the center splash shield as well including the deflectors. look for anything that is loose and that can catch air and flap.

i had the same problem. turned out my brakes were warped and needed new discs. was weird because the booming/shuddering in my car didn't get faster with speed either. also explains/explained to me why i was losing performance. drive like 10 mph with windows down and see if you can hear quite a subtle rubbing sound