Find memory leak in a Ruby on Rails project

I have a Ruby on Rails project with what seems to be a memory leak. It keeps using more and more memory until it crashes. Dumping the amount of objects per class using ObjectSpace I've found this:

Name                                                              Count
-----------------------------------------------------------------------
String                                                           649476
Hash                                                              59695
Array                                                             39407
ActiveSupport::Multibyte::Codepoint                               19337
FileNode                                                          17134
Time                                                               3391
Regexp                                                             1944
ActionController::Routing::DividerSegment                          1743
Proc                                                               1597
Gem::Version                                                       1545
Class                                                              1503
Gem::Requirement                                                   1479
ActiveRecord::DynamicFinderMatch                                   1021

I believe FileNode is the problem. It's a model. Any ideas how to find where the references to the 17k FileNodes are being kept?

This is using Ruby 1.8.6 and Rails 2.2.0. Upgrading is not an option unfortunately.