I had the same issue and had to replace the intake gaskets behind the plenum. I found that while some workshops will happily quote you and say its possible to do in the car, the engine really does need to come out to do it properly, which in turn opens the door for other work to be performed at the same time.
UAS did the work on my car, removing the engine, fitting a Tomei metal intake gasket kit, new timing belt, new water pump and some other things at the same time. You will find there isnt a great difference in labour removing the engine, as trying to remove the plenum with the engine in is an absolute biatch of a job to say the least.
Id recommend the Tomei gasket kits though - give UAS a call if you have any questions.
Nick