1100 ~ 1200 ~ 1300 rpm Jitter, vibration, noise form Driver side exhaust pipe

[WillZhai]

My car used to get a lot of low frequency noise around 1200rpm, which some people may not be sensitive to. In the past few months the noise has become quieter. Then last month I noticed a strong vibration on the driver side at the same rpm. The vibration almost disappears after 1200rpm. I looked under the car and found that the driver side exhaust pipe was shaking up and down like crazy. To diagnose the problem I jacked the exhaust pipe and re-accelerated to 1200rpm, and the vibration completely disappeared. This allowed me to rule out the engine mounts, if the mounts are old then supporting the exhaust pipe obviously cannot prevent the vibration. I took the car to a car shop to weld a new flex pipe. After confirming that the terrible vibration at 1200rpm was effectively suppressed, I found that there was also a slight vibration at 1800 (1700-1900) rpm, and I am worried that the passenger side exhaust pipe also has a natural resonance range and it vibrates differently than the left side. I have already spent 220cad to weld the left side and I will be very sad to spend 220cad to weld the right side. Also note that the low frequency noise returned after the replacement. I suspect that the engine mount and flex pipe are aging at the same time and can no longer suppress vibration, but the engine mount is not completely useless and needs to be replaced, so the vibration is transmitted to the exhaust pipe and causes strong resonance. This low-frequency noise is really disgusting and very strong. I suspect that this also accelerates the aging of the mount. I saw someone in the forum fixed the vibration by replacing the mount while the other one did not help. This also indirectly proves that these three parts may be experiencing simultaneous aging.
If anyone has similar experience, please discuss with me. If you replace the flex pipes on both sides, can you still feel the 1200 and 1800 vibrations?