Loud tappet noise in renault clio engine

i own a renault clio 1.4 Rt 1997 with 35k miles on the clock.the engine runs very well, almost feels like new apart from the exceptionally loud tappet noise.this is always the same,and if the car is run hard it will get worse and the engine lights on the dash flicker as if about to stall.i wondered if there would be anything a garage could do to solve this problem, or indeed anything i could do myself with minimal experiance in cars. thanks.