tapping sound when engine cold and wam?

i have had a tapping sound on my frontera 2.2 petrol for the last 2 years ,no car fitter i have spoken to seems to know the problem.
The sound seems to come from the exhaust side of the engine the engine has done 82000 miles and serviced 3 times a year,
Now my ABS light keeps flashing on and off when i reach 40 mph and over. P.S. had new camshaft follower put in 2 years ago.

ken wood
January 2009