When I started my car the other day, the engine made a different noise. Nothing in particular, but the general humming noise the engine makes changed and now sounds lower – especially when driving at low speeds (under 40 MPH).

To an extent, the sound is more muffled, which isn’t necessarily the most accurate word to use, but it is the closest word I can think of to describe the change in noise.

My car is a 1998 Subaru Legacy Outback in good shape (engine rebuilt in 2008). Any advice or general comments are welcome! I do plan on taking it to a mechanic, but prefer to do my own research before I take it in.

No, I can not use an onomatopoeia to describe the humming noise. It's like mmmmmmmmmmmmmm, but lower in pitch, and kinda muffled.