If you have a Harbor Freight nearby you can pick up a mechanic's stethoscope for about $6 which can help isolate the source of the sound. You can also find them at NAPA and other auto parts houses for a little more.

If the sound is coming from the head then it's likely the hydraulic valve-lash adjusters, which have a published known-issue of sticking from varnish build-up. The typical "fix" is the use of 0W-30 or 5W-30 Mobil-1 which will remove the varnish over a period of time. The adjuster could also be failing (unlikely at your mileage, but possible), which in a worst-case scenario would cause very rough running as the affected valve stuck open.

Ldub's exhaust shield clatter is another valid possibility, and a while back Johnnyapollo had a noise that would only happen while in drive that turned out to be the exhaust making contact with the right side suspension link.

How long have you had your VX and been using Mobil-1?