First on the slip.... By "slip" do you mean your rear cassette just seems to spin freely as you spin the pedals or it "slips" as in when you shift (rear or front), the chain just seems to jump on/off the gear you are shifting into. Resulting on having to move the shift trigger one more time to get it to catch? If the first one, then you may have issues with the cassete/rear hub. I think you mean the second one which could be several things. If the teeth on your crank rings look like they are shark fins, then they are probably worn out. If they are bent (left/right) sticking out a bit, then you may just need a quick mallott put them back. If you have a chain check tool, it will tell you if you need to replace the chain. Are you lubing it and how often? Are you cleaning all the greese/mud/dirt from chain, rings, cogs at lest once a week or so?..... Give more detail.
On the low hum/whine noise... Is this when you are braking or when you are just riding? Turn the bike upside down and spin each weel with your hand. Can you hear the noise? If so, is it a constant noise or it goes and comes back? Your rotor may be bent and is rubbing against the brake pads? Or your rotor is dirty with oil/greese and is squeaks loud when you brake... depending on when the noise is heard. As you spin the wheel, look at your rim and your disc rotor to make sure it is rotating without wobbles. If wobbles, then your rotor or rim needs to be true or is warped. Once you tell us which type you have, we can tell you several adjustment options.
On the lever being tight.... Do you mean that when you press it to brake, it doesn't take much effort to engage right away? Or do you have to press it real tight, almost against the handle bars to get it to engage? In either case, depending on the type of brakes/brake handles you have, it needs to be adjusted. Let us know what type of bakes and brake handle you have. They all have different adjustment options.