whenever i change into 4th or 5th under pressure the whole car shudders and rattles what the hell is it. its quite disturbing. it rarely does it when driving nomally would it be an engine or gearbox mount or maybe an alignment prob where do i start.

You start by explaining what you mean by pressure?

pressure? the good ol flatchange or when i stand on it to overtake a car in fourth from bout 2500rpm it not a wheel alignment problem if that helps

Sounds like uncontrolable wheel spin!

Nah but whats the rattle sound like?
( what would you compare it to?)

sorry bad description not so much a rattle like what your after.(may be my brain rattling) maybe shakes the whole car shakes like a wheel alignment prob sometimes its so bad the steering wheel shudders but mostly it just the car that shudders

are the wheels balanced ?
are the shocks ok?
is the clutch had it?

shocks and wheels should be okay but ill get em checked. while idling if i put the clutch in the revs rise(outa gear) when i let it out the revs fall would that have sumfin to do with it. sorry im not terribly mechanically minded. only got a little schmick

Already sent this but don't know where it went!

when car is out of gear and idleing the revs shouldn't change much whether the clutch is in or out!

this doesn't sound like it would cause the other problem!

sounds like the transmission is on its way out. Maybe the tailshaft bearing.

If it doesn't happen in 3rd at at the same kph, but happens in 4th and 5th (at the same kph, the driveshaft, diff and tires at going the same speed).

Or if it doesn't happen at the same RPM in other gears.

It could be a harmonic problem ... is the tranny mount rubber in good shape, or is it broken or loose?

WEll my datsun use too doo the same thing but in 2nd gear then later i blew second gear so i would say youre tranny is going out.

ps. this may sound stupid but could the harmonic balancer have anything too doo with it?