YODAONE

Discerned clunking/rattling noise from the front grille assembly on 1998 and 1999 LS400s...

Two of the three, and three of the three plastic bosses(looks like a round plastic tube) in the 2nd vehicle use self-tapping screws to secure the plastic grill to the chrome frame, had separated from the plastic grill portion were contributory.

These pieces were recoverable because they would not allow the screws to back out and were still attached to the 3 grill fasteners on disassembly(binder tapping screw # 90164-40059 on attached Lexus parts diagram)

I used JB weld, to reafix the bosses, in correct orientation/alignment, and the base of the plastic boss which had not seperated to fortify it. (If yours are intact, suggest you fortify these bases with JB Weld before reassembling)The unbroken boss was used for proper orientation of the other two..important.

Also critical is to turn the self-tapping srews in the opposite direction of tightening, until you feel the screw threads catch in the tapped trace, then tighten...if you forget this detail, then you may shear off the brittle plastic bosses while tightening. Go light on torque pressure and speed.

The rubber "Radiator Grill Protector" , 53119B in the parts diagram was also tired and expired. (Lexus P.N. 53119-50030)
After acquiring this new part and removing the old protector from the metal grill surround requires use of a high-quality gasket remover....a plastic or wood scraper is helpful, but unless you remove all remnants of old protector, the new rubber grill protector(it's pressure-sensitive adhesive is exposed by removing peel-off backing) will not fit and adhere properly. It is also imperative to rinse the metal grill surround of all gasket remover, and thoroughly dry before applying the new gasket. Pay particular attention to how the old gasket was applied before removing...or better yet...take pictures as you go.
Lexus sent the replacement part folded and jumbled in a parts bag...spread the part out, allowing it to flatten for an hour or two before using.

Other parts to replace;

QTY 2 Cushion #90541-08020
QTY 3 vinyl "Washer plate" (or 1 PKG) # 90202-06004 (used chapstick to postion in place)
QTY 2 plastic

Grommets "Pipe Radiator Grille" diagram # 53129A, Lexus #53129-50010
Qty 5 machine screws P.N. 90119-06660. Shows as discontinued..Lexus may offer substitute...however Lexus used steel phillips head which were badly corroded thus requiring replacement. Fasteners are M4 X 9mm ...substitute stainless M4 X 10 or M4 X 8 from McMaster-Carr.

All parts, including gasket remover, but excluding JB Weld, was about $30