discovery 300tdi sounds tappety and burnin small amount of oil!!!

my 300tdi has recently had a full head rebuild (skim, pressure checked, new stem seals etc) then had to have rocker shaft stud helicoiled due to stripped threads at fair expense! runs almost right but still a bit tappety almost and is burning a litre of oil to 450miles roughly the nly things i can think of is either stem seal failed or rings worn/damaged or turbo seals on way out! can anyone enlighten me or any other ideas! cheers