Resonating HVAC Ducts

Hi everyone, first time poster here. Hope everyone is doing well. I'm writing because I've moved to a new project space for sound editing and some mixing. It's a 8 (w) x 9 (h) x 20 (l) room with drywall and concrete/tiled floor (basement of an NYC building). I'm in the process of getting bass traps and panels in place, but the room has one issue that I'm not entire sure how to resolve: the HVAC ducts resonate badly at 133 Hz, and is pretty loud at a seated position around 36-37". The resonance is exacerbated at higher and higher SPLs. I've attached a photo of the room, and there are two vents. I removed them and when i tap the elbow extensions, they resonate right around 133 HZ (i'm playing a 133hz sine wave, and when i tap on the metal, there's a spike in level, and when i press my hand on a side, the resonance diminishes a little...not a lot, but noticeably, maybe a 2-3 dB, but it's hard to tell since my ears are right up against the ceiling). The ducts are lined on the inside EXCEPT the two elbow extensions. I'm not sure how to exactly pinpoint the very problem, but I think one step would be to stop the ducts from vibrating. Any suggestions on how to add weight/mass or stop vibrations would be greatly appreciated. On a side note, the ceiling seems "loose"...in other words, if I hit the ceiling, there is a little "give". This is probably a factor as well, but I think the ducts might be the bigger culprit.

My thinking, at this point, is that the ducts are simple a path and the perceived frequency disturbance could be coming from within the room and riding inside the duct from the monitoring position to the back wall position...or vice versa. Since this is not a rectangular room (noting the back enlargement) and the treatments are minimal, it can still very well be coming from something/somewhere else. What are the precise measurements of this room?

Hi, thank you for your response. Since I've taken that photo I've added corner traps on the front two walls, 2 4" clouds with about 3" air gap, etc. I still have 8 2" and 4" absorbers but have been hesitant to place them until I can pinpoint the root of this issue. I'll post the exact measurements when I get back to the space.

I just took some measurements: The front part of the room is 95.5" wide and it expands to 111" wide The front part is 140" deep, followed by 15" of an angle, and the total depth to the back is 243" The height of the room is 100.5" There's also an HVAC/furnace closet in the back, and perhaps this room serving as a resonator into the ducts? It's 69 (l) x 50.5 (w) x 100.5 (h). My calculations show that 134 Hz is a severe issue for the closet, which is close to the 133 Hz resonance that I'm hearing. Any insight would be much appreciated!