When I put my foot flat in my v6 3.5L Magna Sedan, after I hit about 80-90 the engine sort of makes a slight shreik.
sounds like something is sliping, also sounds kinda like a beep. seriously don't know whats wrong, but the cars performance seems pretty good. it usually happens after the change from 3 to 4 i think. Anyone having the same or similar problem and what can be done to fix it.

Most likely a slipping belt (Alternator/air con, or power steering) - Have a mech check it out.

It sounds like AC belt slip. Youll find that if u do the same thing with the AC off it wont do it. Its a fairly common thing in magnas to have the AC belt slip a tiny bit during a gear change when giving it a bit.

BTW if you are changing into 4th gear at 90kmh with your foot flat to the floor then youve got more serious issues to worry about than belt slippage.

Freaked me out too - I had my car for over 3 months before I noticed it because I hadn't used the air con previously!

For me, during hard acceleration the AC belt slips during gear shifts - I thought the car was pretending to have a tubo's blow off valve!

lol haha. oh and yer i just check it its not 3 to 4 its like in second gear and again in third.
im gona take to mechanic tmrw then.
does it cause any loss of power

Shouldn't, but it means the belt isn't tight enough or it's wearing out. Better to get it looked at rather than have it break and cause some damage.

More than likely it is a Air Cond belt or pulley, mine did the same thing when Aircon was on, Car yard is going to fix it soon under warranty.

Mine has done it since i got the car more than 2 years ago, hasnt caused any problems and the mechanic has looked at it and says it looks fine.