There are 2 torx screws that hold the handle onto the liftgate that must be removed. The tool Steelcity suggested may help....I have one, but I actually just pulled the panel off....just call me Clark Kent.
Now, I have had a rattle, and I have a second one. The first rattle was unfortunately a rear window that lost it`s seal. The one I have now is more elusive. GM has issued a TSB regarding this. Not sure if it applies to your year, but something to check out. Here`s the TSB:
File In Section: 08 - Body and Accessories
Bulletin No.: 03-08-59-002
Date: April, 2003
TECHNICAL
Subject:
Rattle in Liftgate Area While Driving
(Install Insulator)
Models:
2001-2003 Chevrolet Venture
2001-2003 Oldsmobile Silhouette
2001-2003 Pontiac Montana
Condition
Some customers may comment on a rattle noise coming from the liftgate area of the vehicle while driving.
Cause
The rear radio speakers mounted in the rear lingate may have a loose fit. This may cause a rattle noise while driving the vehicle.
Anyways, there is a second TSB as well, but with slightly different symptoms that you describe:
File In Section: 08 - Body and Accessories
Bulletin No.: 02-08-59-003
Date: October, 2002
TECHNICAL
Subject:
General Squeak and Rattle Guide
Models:
2002-2003 Chevrolet Venture
2002-2003 Oldsmobile Silhouette
2002-2003 Pontiac Montana
Condition 1
Loud clunking noise at the rear of the vehicle.
Cause
There may be loose or cross threaded rear shock bolts. Refer to the illustration above.
Correction
Clean/tighten or replace the bolt.
Tighten
Tighten the bolt to 85 N.m (63 lb ft).
Condition 2
Loud clunking noise.
Cause
The exhaust may not be connected to an exhaust hanger. Refer to the illustration above.
Correction
Reinstall the hanger.
Condition 3
Loud "Clunking" noise at the rear of the vehicle.
Cause
The rear coil spring may be contacting the body.
Correction
Assure that there is no contact to the body. Reposition the spring if there is contact to the body. Refer to the illustration above.
I do have a couple of pictures if you want to make repairs yourself.
Ian