Question about an engine noise

[E300DW]

I was wondering if anyone would be able to help me diagnose a noise in my 95 E300D. When I am on the interstate at speeds of about 70-80 mph everything sounds fine. That is, until I start to go up a hill and must put a load on the accelerator. I get a "knocking" noise from under the hood. Once I peak the hill and let off the noise goes away, and all sounds fine. It may be my imagination, but it seems worse when the temperatures are colder. I would appreciate anyone who can help me shed some light on this. It has been happening for awhile, and I'd like to try to get her fixed. Thanks!