How about a mechanical hammer.
Hit the tube, and it will vibrate at its resonant frequency.
Then place a microphone very close to the surface (1-2 mm distance) and measure the frequency. Unless you're at a pop concert you shouldn't have much issues with ambient noise. Just tell the other workers in your lab to shut up for a moment if that is still too much.
Your hardest problem may be to have the pipe move freely while suspended as indeed anything attached to it, including the attachment point to stop it from falling on the ground, will influence the response.