If you can hear it then it is too late for grease.
I have always suspected that the reason there is no service interval for greasing front wheel bearings is that the cost of such service would dwarf the cost of the bearings.
If you were to get a manual you might be scared off by all the special tools indicated. Actually the job can be done with simple tools. Its as straight forward as a Chevrolet and may even use the same bearings. The bearings can be bought from any parts store under the number "Set 3" and "Set 5". They are the same as numerous manuafcaturuers use. You will be surprised how cheap these bearings are.
The brake caliper will need to be removed, leave it connected (so you won't have to bleed the brakes afterward). Hang it with a coat hanger (not the rubber hose). Loosen the allen screw that locks the pinch nut holding the outer bearing (under the grease cap). Unscrew the pinch nut. Pull the assembly from the spindle, wasn't that easy! It will be in 75% of the instances. If you do both sides there is great likelyhood that one of the four bearings will stick to the spindle - not so easy).
After the outer bearing is removed I usually reinstall the pinch nut and then use it as a tool to pull the inner bearing and seal from the disc/flange combination. To do this you grab the disc on both sides and give a sudden pull. If the rear bearing isn't stuck the whole assy will come out about four inches and hit the pinch nut. The impact will drive the rear seal from the flange and leave the inner bearing and seal hanging from the spindle, discard both.
Unless a bearing sticks the hardest part of the job will be to remove the pressed in race of the bearings from the hub. You CAN"T just replace the rollers. The whole bearing (two pieces) must be replaced. There are special tools to do this but a long punch and hammer and tapping all around the circumference will work it out. Use the old race and a hammer to tap in the new race. DO NOT impact the face of the bearing. Be sure the race is fully seated.
Grease the bearing, NOT JUST EXTERNALLY. You must push the grease through the inners of the bearing. Without tools this requires a hand full of grease. Take the bearing in other hand and push the bearing sideways into the handfull of grease, shoving the grease sideways between the rollers until it comes out the other side.
Place the rear bearing into the hub and tap in the new seal. Install onto the spindle, wasn't that easy..... sometimes it won't go. uh-oh. Don't have much to say if it don't except to clean the spindle and try over concentrating on getting the bearing straight as you push in.
Place some grease in the middle but do not fill, MB would tell you to use xx grams of grease. Basically about half full. Install the outer bearing. Tightten the nut with a pair of pliers while turning the hub till fairly tight, somewhat more than hand tight; this is to be sure the bearing is fully seated. Back the nut off and tighten by hand. The bearing is not to be tight. It actually is supposed to be loose by about a few tenthousandths of an inch. Less than .001in. This can not be felt and even can hardly be measured with all the grease in the bearing. It is a thing of experience and feel.
One technique is to mount the tire and hit the tire with your fist as you tighten the nut. There is a rattle sound that occurs with each hit. When you reach the point of proper tention the rattle sound diappears. Unfortunately as good as this works it should be demonstrated for proper understanding. Just remember the bearing is NOT to be tight. That is the reason for the pinch nut. It can be tightened and will stay in place without physically being tight against the bearing as a bolt would be when tightened.
Bosch Master, ASE Master, L1
33 years MB technician