hi can anyone help???

hi i've bought a 94 maverick 2.7td last week which had been stood for 12months. i bought it because i had a 94 terrano before and it was great trouble free motoring and very useful. however-it appears that this one isnt so great, altho it shows 49000 miles, and has mots to back up mileage for past few years, i had to fit a turbo to it this week, that stopped the smoking, took it for an mot today, passed fine, and on the way back at 60mph, it started rattling intermittantly like a 'tappet' noise, but this got worse after within minutes and is now constant. it still starts fine and the oil pressure light goes out when started up, and i took the rocker cover off and nothing is obvious. any ideas?? is there any common faults that would cause this? im desperate to fix this if it is!! contact me at paulbarton1@live.com
cheers