I have a 1998 disco 1 300tdi auto. Just over 230000km on the clock. I'm hearing a tapping noise under the bonnet that increases with revs. My thoughts are that its either the vacuum pump or the exhaust manifold gasket. Hopefully not the head gasket. My problem is I cant tell exactly where the noise is coming from. Any tips on how to source the sound? I'm not keen on the vacuum pump failing on me if that is indeed the problem!