The engine is making a loud noise towards the bottom of the crankcase. but when i raise the throttle it goes away

Should idle between 500-600. Clean your throttle body with the battery disconnected to help with the fluctuation. The roaring very well could be your fan clutch. Fan clutches are a known problem with our platform. If engaged, will sound like an airplane.

First do an oil change , 5W/30 , and add Wynn thin upper cylinder oil to crankcase .

The rattle is caused because the rpm is lower. When stopped in drive with one foot on the brake, press the throttle down slowly until the vibration stops and the noise goes away. Cause? Weak or bad motor mounts, loose bolts or parts.

Hard to say over the internet but I will try to explain. If the noise is a distict KNOCK KNOCK and it sounds like it is coming from the bottom of the engine, then it most likely is a rod or main bearing. If it is a TAP TAP noise that sounds like it's coming from the top of the engine, then most likely a valve lifer is failing/failed.
the fact that it goes away when the car is warm, makes me lean towards a bad lifter. Normaly if a bearing is bad and knocking, the noise doesn't go away.

The KNOCK noise will be somewhat hollow sounding and the TAP TAP sound will be more solid. If the KNOCK KNOCK is solid, then a rod or main bearing has most likely failed to the point that a new/reman engine will be needed.

if the noise is coming from the bottom end of the motor then you have either bad connecting rod bearings or bad crank bearings.if when you snap the throttle you hear a double knock then its connecting rod bearing if not then its crank bearing.a collapsed piston can sound low in the engine and its usually a dull knock noise.if you have the engine running and the noise is there and you pull off one plug wire at a time(and reconnect it after checking) and the noise goes away or mostly goes away with the wire unhooked then the cylinder that the noise changed is the cylinder that has a bad piston.

Take the belt off to see if it goes away. If it goes away, one of the accessory pulleys is the problem. If it still makes the noise with the belt off, you have a problem internal to the engine. Check the oil to see if it is milky, like water has gotten into the crankcase.

could be the belt tensioner,

The oil trap box is plugged up. You need to remove the intake manifold and replaced the oil trap box. It is the large black box mounted against the block just behind the a/c compressor. While it's apart check the nipple that goes bolts to the underside of the intake manifold. Use pipe cleaner to make sure it's clear. Also replace both seals for the oil trap box and of course the intake manifold gasket and it's a good idea to clean the trottle body while it's all apart. Use either BG air intake cleaner (available from and BG dealer or most Volvo dealers) or Berrymans B12 carbuerator cleaner. Spray it on a rag and wipe the inside of the throttle module to clean it. don't spray to much directly onto the throttle blade, it is possible it will damage the seals inside the unit.

Hope this helps.

heads are bad

I would guess it is the electronic fan clutch. It will cause the fan to run too fast (very loud) and the light will come on sometimes.