lifter noise on acceleration

[mikethepole]

hello

I own a low mileage 2004 Vibe GT. the car has been regularly GM serviced its entire life. it just passed 100,000 km (60,000 miles).

I noticed recently that when I accelerate, I hear some strange noise by the engine. it seems like lifters or a cracked exhausted manifold. but, when I stop and rev the motor, I do not hear that noise. it seems that the only time it makes noise is when in gear and under load (acceleration). the noise does not get louder as the revs go up.

what could this be? some valve or lister adjustment? a cracked exhaust manifold? but if so, why doesn't it make noise when revving it while not in gear

I understand this is a vague description but I simply don't know how I can be more detailed.

any feedback is welcome

thanks