My car is a 2001 Pontiac Firebird with a V6 (3800 Series II (L36)) and a manual transmission.
These are all at a stop:
When the car is cold, I don't feel too much vibration when sitting in it. (I mostly notice the vibration in the seat.) However, the vibration that I do feel is similar to what I feel when it's hot, just not as powerful/strong.
When it's hot, it will get a bit worse.
If it's hot and I've driven it for a bit, I will feel considerably more vibration. Edit: Maybe I had the A/C on? That adds vibration, even if I haven't driven it.
My understanding is that vibration in the seat is usually from mounts, but that mounts will be worse when cold - not when hot?
When I increase my RPM to 1400, my car feels normal. The rough vibration gradually goes away as I increase RPM from idle (idle is ~690rpm when warm.)
Here's a video of my harmonic balancer, if it helps: https://www.youtube.com/watch?v=YxjNI2u4dNo
No check engine light. A few DTCs were found with my OBD scanner, but I think they were all related to when I ran the starter with a couple fuses out (I recently replaced my steering rack and steering pump.)
What's likely the cause?
Some info from Torque (everything passed) :
Test report:
------------------
TID:$05 CID:$01
- Rich to Lean sensor switch time(calculated)
Max: 65,535
Test result value: 14,400
PASS
----
TID:$05 CID:$02
- Rich to Lean sensor switch time(calculated)
Max: 65,535
Test result value: 15,999
PASS
----
TID:$05 CID:$03
- Rich to Lean sensor switch time(calculated)
Max: 65,535
Test result value: 9,583
PASS
----
TID:$05 CID:$04
- Rich to Lean sensor switch time(calculated)
Max: 65,535
Test result value: 19,166
PASS
----
TID:$05 CID:$05
- Rich to Lean sensor switch time(calculated)
Max: 10,240
Test result value: 1,408
PASS
----
TID:$05 CID:$06
- Rich to Lean sensor switch time(calculated)
Max: 4,480
Test result value: 1,088
PASS
----
TID:$05 CID:$07
- Rich to Lean sensor switch time(calculated)
Min: 65
Test result value: 305
PASS
----
TID:$05 CID:$08
- Rich to Lean sensor switch time(calculated)
Min: 55
Test result value: 300
PASS
----
TID:$05 CID:$09
- Rich to Lean sensor switch time(calculated)
Max: 27,904 Min: 4,096
Test result value: 10,496
PASS
----
TID:$05 CID:$0a
- Rich to Lean sensor switch time(calculated)
Max: 850
Test result value: 588
PASS
----
TID:$05 CID:$41
- Rich to Lean sensor switch time(calculated)
Max: 65,535
Test result value: 14,400
PASS
----
TID:$05 CID:$42
- Rich to Lean sensor switch time(calculated)
Max: 65,535
Test result value: 15,999
PASS
----
TID:$05 CID:$43
- Rich to Lean sensor switch time(calculated)
Max: 65,535
Test result value: 9,583
PASS
----
TID:$05 CID:$44
- Rich to Lean sensor switch time(calculated)
Max: 65,535
Test result value: 19,166
PASS
----
TID:$05 CID:$45
- Rich to Lean sensor switch time(calculated)
Max: 10,240
Test result value: 3,264
PASS
----
TID:$05 CID:$46
- Rich to Lean sensor switch time(calculated)
Max: 4,800
Test result value: 1,984
PASS
----
TID:$05 CID:$47
- Rich to Lean sensor switch time(calculated)
Min: 55
Test result value: 205
PASS
----
TID:$05 CID:$48
- Rich to Lean sensor switch time(calculated)
Min: 55
Test result value: 204
PASS
----
TID:$05 CID:$49
- Rich to Lean sensor switch time(calculated)
Max: 28,672 Min: 5,120
Test result value: 13,312
PASS
----
TID:$06 CID:$35
- Lean to Rich sensor switch time(calculated)
Max: 63
Test result value: 30
PASS
----
TID:$06 CID:$41
- Lean to Rich sensor switch time(calculated)
Max: 279
Test result value: 54
PASS
----
TID:$06 CID:$55
- Lean to Rich sensor switch time(calculated)
Max: 64
Test result value: 31
PASS
----
TID:$07 CID:$4d
- Minimum sensor Voltage for test cycle(calculated)
Max: 33,873
Test result value: 30,809
PASS
----
TID:$0a CID:$04
- Sensor period(calculated)
Test incomplete or dependant test failed
--
TID:$0a CID:$05
- Sensor period(calculated)
Max: 220
Test result value: 68
PASS
----
TID:$0a CID:$07
- Sensor period(calculated)
Min: 500
Test result value: 500
PASS
----
TID:$0a CID:$42
- Sensor period(calculated)
Max: 32,868
Test result value: 32,772
PASS
----
TID:$0a CID:$48
- Sensor period(calculated)
Max: 32,878
Test result value: 32,770
PASS
----
TID:$0c CID:$60
- Misfire counts for last/current driving cycles(calculated)
Max: 34,006
Test result value: 21,039
PASS
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I was going to give a few images, but I can't.
One image from torque: