The immediate symptoms of the problem have been reported to include: "suspension failure, partial or complete wheel detachment, steering anomaly, and/or sudden vehicle disablement," according to the NHTSA summary of the complaints. Several of the failures have occurred in traffic, although the majority were discovered during mechanics' inspections.
Buyers of Sonatas with model years 1999 through 2002 have reported 'fist'-sized holes in the frame according to the NHTSA website. The problem either is more prevalent among the earlier cars, or takes several years to manifest. Of the 40 complaints received by the NHTSA, 19 are for 1999 models, 16 for 2000 models, 3 for 2001 models and 2 for 2002 models.
The focus seems to be on the right side of the frame, especially the right rear corner where the suspension mounts attach. Some reports note the engine cradle, cross-member or front sub-frame, however, so it does not appear to be exclusively localized to the suspension mounts.
A contributing factor is likely the addition of road salt, since 35 of the 40 complaints originate from the so-called 'salt belt' of northern states that have severe winters.
Via: The Truth About Cars