Single metal-on-metal knock when turning right

There's a metal-on-metal single knock when I turn the steering wheel right sharply.

Observations so far:
- Seems started after I changed the head gasket last summer. May be a red herring - I started to care for my car, replacing stealership mechanics with myself at about the same time.
- Likely coming under the hood, driver's side.
- Reliably reproduces at medium speeds, especially when entering a roundabout at 15-20mph. It's hard to jerk the steering wheel at high speeds without loosing control.
- Isn't reproducible at low speeds, approximately below 10mph no matter how hard you try to jerk the steering wheel left and right.
- Doesn't seem to be reproducible when turning sharply left.
- Not reproducible when starting and stopping without any turns.
- Sometimes I hear it when driving - especially when a wheel goes into a pothole during a descent.

What parts have been changed (all Toyota OEM) already:
- Struts + mounts. Except springs (they're fine).
- Tie rod ends
- Front Stabilizer Link Assembly + bushings
- Inspected steering wheel attachment, tightened a bit the nut to the spec - no change.
- Inspected steering rack - zero play.
- Bottom engine/trans mounts visually look ok - no cracks.
- Haven't noticed anything unusual (like looseness) with the top engine mount on the passenger side when doing the head gasket job.
- Thought it could be the devil's EGR cooler that I decided not to put on - got a new stud and a new nut and managed to bolt them back on at the bottom of the cooler - no relief.

I'm suspecting the top transmission mount next. How easy is it to replace without taking the entire engine out?