Vibration at idle, much worse after warm

Question

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
----

I was going to give a few images, but I can't.

One image from torque:

Answer 1

Although, your point about cold engine mounts applies, I would doubt they heat up enough to significantly affect vibration transfer to the interior.

Instead, what you might look at is increasing your idle speed a bit if that's easily doable. Every engine has a range of natural resonance, the range where the engine simply vibrates more than in other ranges. Below certain threshold, engine would vibrate no matter what. Quality of fuel and age of the engine would also matter.

Reason why you experience it less when cold, is because cars add more fuel mixture and increase RPM when cold to warm up quicker. Once it settles at the normal idle mark at 690 RPM, it starts to vibrate.

Also, check your engine mounts to see if they are still intact. Sometimes mounts might deteriorate and engine would be resting on metal frame incresing any kind of vibrations.