Should I send it back for warranty service?
Yes. Why? Three reasons.
If it is on warranty, any modifications or breaking of the seal will void the warranty.
If they decide that since you are responsible for the damage, that they are not responsible for repairing it (IE, your warranty covers manufacturing defects, not user ones), you are back in the same boat of looking to repair it yourself anyway... which is where you started.
I gently dropped it one day
I have heard this way too many times. I only dropped it a little or maybe I didn't drop it very far or how about I just tripped on the cord, and it came off the desk... not that hard though. No one ever thinks that the sudden impact has anything to do with the sudden performance drop of their complex and expensive electronic device. Do you know why things like dropping a laptop aren't covered by most warranties? Because they are common, and they are virtually guaranteed to damage something to the point of requiring replacement.
Now I know... if a drop isn't covered by a warranty, why send it back? Well.... I know it's not all that ethical, and it's definitely not honest, but if you omit the fact that you dropped it and the laptop does not show any physical damage from the impact, Lenovo might just repair it on warranty. Unless you have money to burn, having Lenovo pay to replace a hard drive (or god forbid something worse) is better than you paying to replace it... even if you were the cause of the damage in their eyes... aka the drop was not a manufacturing defect.
Did you know that your hard drive has a warning written on it not to subject it to exactly this kind of shock?
There are a couple of possibilities, and in all honesty the laptop should be taken into a shop at the very least. But, since it is on warranty, that shop should be Lenovo.