Memory Pressure in error log

Hello Experts,

iam seeing the below error in our errorlog. we have two production databases on this server, but this setup is there from last 5 years. we are seeing these errors very recently.

2015-03-24 09:02:21.640 spid1s       AppDomain 4 (mssqlsystemresource.sys[runtime].5) is marked for unload due to memory pressure.
2015-03-24 09:02:22.060 spid18s      AppDomain 4 (mssqlsystemresource.sys[runtime].5) unloaded.

any idea why we are getting these errors? when i did a bit of research, i fount on the internet as this will occur due to user defined data types. but we are using these from more than 5 years.

please help.

Thanks in Advance

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Cheers, Vinod Mallolu