Its hard to pinpoint any problem relating to sound when there are so many different possible problems. If you can record the sound with your phone or camera and post the link on this site to youtube that would draw a better response.
Some possible abnormal noises from an engine can be rotating noises or reciprocating noises. A lot of noises sound similar from each of the following so you will need to ask an experienced ear at a garage if you are new to this.
1. Failed rod bearing
2. Bad lifter
3. Bad cam lobe
4.Cracked piston/ loose wrist pin.
5. Bent valve/ sticking valve in valve guide.
6. Cracked cast exhaust manifold
7. Blown head gasket
8 Old vee belts can develop a cracking sound.
9 Old bearings in water pump. grinding sound.
10 Loose camshaft thrust plate. ( cam moves back and forth and hammers lifters)
There are more than this which is why imo it is important to save yourself hours of chasing and get an experienced ear. Mechanics stethescopes are a valuable tool for pinpointing abnormal noises.