The conversation drifted towards another issue that I'm not primarily concerned about. It's not the noise but the uneven nature of the sound which is bothering me. My mistake, let me add more to it. The fan is constantly ramping up and down with a strange whistling noise that my D3 units don't make. I also have some more information to add to it which might help. I found out when is the fan making the noise.

Please take a look at the attached image:


https://imgur.com/a/cyANY

You see 3 'yellow' columns which signify the duration for which my S9 was rented on miningrigsrental.com

From the right, the highly fluctuating hashrate graph is when the S9 is being noisy. At this point, all the hashborads are under 78c. During the rented duration when you don't see the fluctuation in the graph, the S9 is running smoothly.

There's nothing I'm doing from my end that might cause this uneven hashrate.