My rear wheels have been making rumbling noise. According to my past experience I know it is the axle nuts that are probably loose. So I finally checked them today. Sure enough they are loose. I tightened them until I could not turn further. I do not have a torque wrench so I am sure I did not apply enough torque.

The questions is I did this once less than a year ago. I turned as much as I could until I could not turn further the nuts. I installed the cotter pins properly I believe. I am surprised to find that they are so loose today. I turned them about 30 degree each then put in new cotter pins again. Is there anything that may be wearing out in the axle? With the cotter pins in there the nuts can not possibly turn back out. Last time I tightened them as much as I could until I could not turn any further. But today I found I could turn another 30 degree further. Does this mean something is wearing out inside? Is there any risk if I drive it as is for now until maybe they get loose again?

Thanks in advance for any expert tips here.