my opinion is the bottom end, sounds very familiar to me as it happened on an old rst of mine, whip the sump off and have a gander. Hopefully im wrong

ZVH, Zetec, CVH, you could spend pennies or thousands on each, jest depends on what you prefer. Here is my breakdown:

cvh - pros, nice and simple, everyone knows then, good power for not much money.....cons, bit long in the tooth, capacity is limited, can be expensive when chasing big bgp

zvh- pros, can be very cheap, can handle great power when built correctly, has a very similar layout to cvh so a pleasure to work on, loads of torque.....cons, need forged pistons for higher boost and correct cr, sometimes head gaskets can be a problem due to surface contact

Zetec turbo- pros, silly power can be had out of these (check out luke's) reliable and can produce big bhp with little engine stress, drivability....cons, can be expensive due to manifolds, management, proper pistons for cr eetc, engine bay is really crampped when installed.

These are only my opinions and im sure someone will shoot me down in flames.lol

hope this is of some help mate