The steel pillar with a mass on top will have a fundamental frequency at which it will vibrate, along with a set of harmonics. If you strike the top of the pillar with a hammer it will ring like a bell. The note you hear is it’s fundamental frequency. It may be that the road vibration is exciting the steel tube. You can do one or all of three things: 1) change the mass of the system by say filling the pillar with dry sand. 2) change the stiffness of the pillar (add stiffening webs etc.). 3) increase the damping (strap/bond hard rubber to the pillar). Lookup ‘Mass Spring Damper Systems’ on the internet for a full explanation. Option 1) also increases the damping. Your other mount will have a different fundamental frequency so, you were just unlucky.
When I was still at school many years ago one would annoy the teacher by pinging a wooden ruler on the edge of your desk – we were doing an applied mechanics experiment Sir!